UC-NRLF 


B      5*1 


John  Swett 


A^"  ' 


H.  ;  w 


I  L 


•ENGINE.— PAGE  198. 


JOHNSON'S, 


NATURAL  PHILOSOPHY, 


AND 


KEY  TO  PHILOSOPHICAL  CHARTS. 


ILLUSTRATED   WITH   500    CUTS;    BEING   REDUCED   PHOTOGRAPHIC 
COPIES    OF    ALL    THE    DIAGRAMS    CONTAINED    IN    THE 
AUTHOR'S   PHILOSOPHICAL   SERIES   OF   INDE- 
STRUCTIBLE   SCHOOL    CHARTS. 


FOR   THE    USE  OF 


SCHOOLS  AND   FAMILIES, 


BY 

FRANK  G.  JOHNSON,  A.M.,  M.D. 
h 


NEW   YORK: 

J.  W.  Schermerhorn  &  Co., 

1872. 


Entered  according  to  Act  of  Congress,  in  the  year  1872,  by 

J.  W.  SOHERMERHORN   &  CO., 
In  the  Office  of  the  Librarian  of  Congress,  at  Washington. 

-   -  .    t.  '" 


LANGK,  LITTLK  &  HILLMAN, 

PEINTKRS,  ELKCTROTYPERS  AND  STBREOTYPERS, 

108  to  114  WOOSTKR  ST.,  N.  Y. 


PREFACE. 


THE  rapid  diffusion  of  scientific  knowledge,  and  the  continually  widening  field 
of  its  application  to  the  useful  pursuits  of  life,  have  created  an  increased  demand 
for  new  and  improved  means  of  teaching  the  various  branches  of  Natural  Philoso- 
phy. But  no  want  is  more  generally  felt,  especially  in  common-schools  and 
academies,  than  the  necessity  of  Philosophical  Diagrams,  in  the  form  of  Wail 
Charts,  to  supply  the  absence  of  the  expensive  Philosophical  Apparatus. 

To  supply  this  want  is  the  purpose  of  the  Philosophical  Series  of  the  compiler's 
Indestructible  School  Charts ;  to  accompany  and  explain  which,  and  provide  a  suit- 
able text-book  for  schools  and  academies,  are  the  objects  of  this  volume. 

Before  describing  these,  reference  is  here  made  to  a  Series  of  Charts  prepared  to 
supply  this  need,  by  the  same  compiler,  in  1856  ;  being  a  set  of  ten  Philosophical 
Charts,  3  by  4  feet,  embracing  about  two  hundred  diagrams,  a  large  edition  of 
which  was  readily  sold ;  but,  in  consequence  of  the  engravings  being  destroyed 
by  fire,  no  subsequent  editions  were  issued. 

To  show  the  purpose  of  these  Charts  and  the  favor  with  which  they  were  re- 
ceived at  the  time  of  their  publication,  we  give  the  opinions  of  a  few  of  the  most 
distinguished  men  of  the  age  : 

From  Benjamin  Silliman,  LL.D.,  Prof.  Emeritus  in  Yale  College. 

DR.  JOHNSON'S  Philosophical  Charts  are  well  worthy  of  the  attention  of  all  teachers 
and  learners  of  the  different  branches  of  Natural  Philosophy,  to  which  they  relate. 

The  diagrams,  drawn  in  colored  or  contrasted  lines  upon  a  black  ground,  are  per- 
fectly distinct  and  intelligible,  and  the  large  size  and  handsome  mounting  of  the 
Charts  give  them  a  striking  and  attractive  appearance. 

To  teachers  without  apparatus,  they  must  be  an  invaluable  acquisition,  and  a  very 
useful  one  to  those  who  have  the  instruments. 

Such  illustrations,  as  they  speak  to  the  mind  through  the  eye,  admit  of  indefinite 
extension  to  every  branch  of  Natural  Science.  BENJ.  SILLIMAN. 

From  Rev.  Francis  Wai/land,  D.D.,  LL.D.,  formerly  Pres.  of  Brown  University. 

I  have  carefully  examined  DR.  JOHNSON'S  Philosophical  Charts,  and  think  them  well 
adapted  to  the  purposes  for  which  they  are  intended.  They  will  afford  important  aid 
to  instructors  in  academies  and  schools  where  Philosophical  Instruments  are  not  fur- 
nished to  perform  illustrative  experiments.  In  many  cases  they  will  also  be  of  ser- 
vice even  in  addition  to  any  ordinary  apparatus. 

FRANCIS  WAYLAND. 

Providence,  R.  /.,  Feb.  8th,  1856. 


2  PREFACE. 

From  the  Hon.  Theodore  Frelinghuysen,  Pres.  Rutgers  College,  New  Jersey,  formerly 
Chancellor  of  New  York  University. 

DR.  JOHNSON'S  "  Philosophical  Charts,"  designed  for  the  use  of  schools  and  acad- 
emies, furnish  an  admirable  substitute  for  the  far  more  expensive  apparatus.  These 
Charts,  hung  on  the  walls  of  the  school-room — in  all  of  which  I  hope  to  see  them — 
will  spread  before  the  scholar  a  palpable  illustration  of  the  great  laws  in  Natural 
Philosophy.  He  will  learn  much  of  God,  from  the  works  of  his  hand  and  the  ordi- 
nances of  his  appointment. 

The  small  volume  that  accompanies  them,  and  a  little  explanation  from  the  teacher, 
will  render  the  Charts  one  of  the  most  useful  means  of  instruction. 

THEODORE  FRELINGHUYSEN. 

From,  the  Hon.  Horace  Mann,  President  Antioch  College,  Ohio, formerly  Secretary  Board 

of  Education  of  Massachusetts. 

#  •»  #  -;•:-  •*-  #  *  jn  Sch00is  where  there  is  not  the  Philosophical  Apparatus, 
these  beautiful  "  Charts  "  will  be  an  excellent  substitute  for  it;  and  I  shall  be  glad  to 
ehow  and  to  commend  them  to  such  persons  as  can  best  introduce  them  into  schools, 
and  especially  to  such  as  shall  go  forth  from  our  institution  to  become  school-teachers. 

HORACE  MANN. 

These  Charts  were  made  on  paper,  and  mounted  on  cloth  and  rollers,  in  the 
usual  manner. 

They  were  executed  in  white  lines,  by  printing  the  background  black  ;  which,  it 
is  admitted,  is  the  most  desirable  method,  as  it  renders  the  diagrams  more  con- 
spicuous, yet  easier  for  the  eye.  The  difficulty  of  printing  a  clean  and  pure  black 
on  so  large  a  surface,  however,  made  it  impossible  to  execute  them  with  desirable 
neatness  and  perfection.  This  difficulty  has,  finally,  been  overcome. 

There  are  several  serious  objections  to  the  usual  method  of  making  Charts  and 
Maps  on  paper,  and  then  mounting  them  on  cloth  and  rollers,  which  it  is  desirable 
to  avoid  : 

1st.  As  already  stated,  it  is  next  to  impossible  to  print  a  large  black  ground, 
and  so  give  the  diagram  in  white,,  or  light-colored  lines. 

3d.  Cloth  and  paper,  pasted  together,  do  not  work  well.  In  damp  weather  the 
cloth  shrinks  and  the  paper  swells,  and  vice  versa  in  dry  weather.  This  draws  the 
chart  out  of  a  true  plane,  renders  the  surface  wavy,  and  prevents  it  from  hanging 
flat  on  the  wall. 

3d.  The  tape-binding,  sewed  or  pasted  on  the  edges,  and  the  sticks  nailed  on  at 
top  and  bottom,  render  the  Chart  clumsy  and  awkward  to  handle,  as  well  as  liable 
to  need  repairs. 

4th.  The  cloth  and  paper,  and  the  paste  between  them,  make  the  Chart  so  stiff, 
that  if  it  be  rolled  up  in  damp  and  unrolled  in  dry  weather,  it  is  impossible  to 
make  it  hang  flat  on  the  wall. 

5th.  The  varnish  employed  to  improve  and  protect  the  surface  soon  cracks  and 
crumbles  off. 

6th.  They  soon  appear  dingy  and  show  age. 

7th.  The  paste  employed  in  mounting  often  tempts  the  rats  and  mice  to  test 


PREFACE.  3 

what  virtue  there  is  in  schooling  for  them,  to  the  entire  destruction  of  the  Charts 
on  the  first  investigation. 

8th.  Charts  thus  made  are  not  sufficiently  durable  for  school  purposes. 

To  obviate  all  these  objections,  the  compiler  has  invented  and  adopted  a  method 
of  producing  what  he  terms  INDESTRUCTIBLE  SCHOOL-CHARTS. 

TJie  method  of  making  these  Charts  is  entirely  new.  There  is  neither  paper,  ink, 
printing-press,  tape,  rollers,  nor  varnish,  employed  in  their  manufacture.  They  are 
printed  by  hand  in  pure  white  lines,  with  impenshable  oil-colors,  on  enamelled  jet-black 
cloth. 

They  are  as  smooth  as  glass,  as  soft  and  pliable  as  silk,  and  hang  perfectly  flat  on 
the  wall.  They  are  as  durable  as  a  stone  schoolhouse  ;  they  can  be  employed  as  table- 
covers,  scrubbed  with  soap  and  water  for  years,  and  then  be  employed  as  Charts.  The 
background  is  jet-black,  and  far  superior  to  any  ink-printing.  Black  and  white  are 
not  the  only  colors  that  may  be  employed ;  for  any  desirable  color  can  be  used 
for  either  background  or  diagrams. 

Each  Chart  is  surrounded  with  a  highly-colored  border,  giving  it  a  remarkably 
neat  and  lively  appearance. 

The  mounting  consists  of  an  oval  stick  inclosed  in  a  hem  of  the  Chart  at  top 
and  bottom,  thus  avoiding  paste,  binding,  nails,  and  clumsy  rollers. 

The  Philosophical  Series  consists  of  ten  Charts,  each  33  by  54  inches,  and  is 
intended  to  much  more  than  supply  the  place  of  the  series  above  alluded  to,  em- 
bracing, instead  of  about  two  hundred  diagrams,  over  five  hundred,  on  the  various 
branches  of  Natural  Philosophy,  as  taught  in  schools ;  each  diagram  being  care- 
fully drawn,  and  standing  out  in  bold  white  lines  on  a  jet-black  surface,  consti- 
tuting, we  are  confident,  the  most  complete,  most  durable,  and  cheapest  substitute 
for  the  Philosophical  Apparatus  ever  published. 

These  Charts  are  to  Natural  Philosophy  what  blackboards  are  to  mathematics, 
and  what  maps  are  to  geography. 

Every  drawing  is  made  simple  as  possible,  without  omitting  any  part  neces- 
sary to  give  a  clear  illustration  of  the  essential  law  or  principle  to  be  explained. 

Each  diagram  is  numbered,  and  provided  with  designating  letters  sufficiently 
large  and  bold  to  be  seen  across  the  recitation-room. 

The  entire  set  of  ten  is  arranged,  if  desired,  on  a  Sliding  Chart-Rack,  in  such 
a  manner  that  the  whole  set  will  occupy  but  about  four  feet  in  width  of  wall- 
room,  yet  either  one  of  the  set  can  be  brought  to  view  as  readily  as  a  leaf  can 
be  turned  in  a  book. 

These  Charts  are  especially  designed  to  supply  the  wants  of  our  common-schools 
and  academies  which  are  not  provided  with  the  apparatus,  which  they  have  come 
greatly  to  need,  but  are  generally  unable  to  purchase.  In  many  parts  of  the  coun- 
try, the  majority  of  district- schools  are  no  longer  "  common-schools,"  merely, 
where  is  taught  only  spelling,  reading,  and  writing,  together  with  the  primary 
branches,  arithmetic,  geography,  and  grammar ;  but  they  have  come  to  be  acad- 
emies, where,  at  least,  so  much  of  the  natural  sciences  is  taught  as  is  contained 


4  PREFACE. 

in  the  ordinary  school-manuals  on  Natural  Philosophy ;  thus  creating  a  general 
and  ever-increasing  necessity  for  a  work  of  tJiis  kind  as  a  cheap  and  adequate  substi- 
tute for  tJie  Philosophical  Apparatus. 

With  a  set  of  these  Charts  in  the  schoolroom  the  teacher  can  awaken  in  his 
pupils  the  liveliest  interest  for  the  study  of  Natural  Philosophy,  and  fix  in  their 
minds  more  lasting  impressions  of  general  principles  than  by  any  other  means, 
in  the  same  time  and  with  equal  effort. 

While  the  intelligent  teacher  will  be  able  to  make  invaluable  use  of  them,  with 
whatever  text-book  he  may  have  in  his  school,  or  even  without  any  text-book, 
yet,  to  render  the  Charts  more  useful  to  the  cause  of  education,  this  volume,  instead 
of  being  limited  to  an  explanation  of  the  diagrams  of  the  Charts,  embraces  an 
enunciation  and  demonstration  of  more  general  laws  and  principles  relating  to 
the  various  departments  of  Natural  Philosophy  than  are  usually  contained  in 
school-manuals  on  this  branch  of  education. 

The  average  number  of  cuts  or  diagrams  in  text-books  on  this  subject  falls  con- 
siderably below  three  hundred,  while  these  Charts  contain  Jive  hundred  (counting 
two  for  one  in  a  few  instances,  where  they  are  combined  to  save  space),  all  of 
which  are  contained  in  this  volume ;  and,  in  order  to  have  them  appear  as  much 
like  the  Charts  as  possible,  they  are  made  quite  large,  in  white  lines  on  black 
ground,  and  copied  by  photography ;  hence  they  are  reduced  foe-similes  of  the 
Chart-diagrams. 

The  plan  of  numbering  the  paragraphs  has  been  adopted  for  convenience  of 
reference. 

Physics,  or,  as  more  generally  termed,  Natural  Philosophy,  embracing  several 
sciences,  as  Mechanics,  Hydrostatics,  Pneumatics,  Acoustics,  Heat,  Optics,  Magnet- 
ism, Electricity,  Astronomy,  etc.,  many  volumes  would  be  required  to  contain  all 
that  is  known  relating  to  the  various  branches  of  the  subject.  Hence,  within  the 
narrow  limits  of  a  text-book,  only  the  leading  or  fundamental  principles  can  be 
set  forth.  The  field  is  so  large,  and  the  variety  from  which  to  select  so  great,  that 
compilers  of  school-manuals,  differing  in  their  views  respecting  the  relative  prac- 
tical importance  of  the  different  parts  of  the  subject,  have  produced  a  variety  of 
compendiums,  which,  however,  treating  of  the  eternal  laws,  can  differ  only  in  size, 
arrangement  and  classification  of  subjects,  judiciousness  of  selection,  aptness  of 
illustration,  precision  and  clearness  of  statement,  and  embracing  important  new 
discoveries. 

In  these  respects,  the  relative  merits  of  various  schoolbooks  on  Natural  Phi- 
losophy will  be  most  justly  estimated  in  the  schoolroom  by  the  practical  teacher 
and  earnest  student. 


TO    THE    TEACHER. 

To  make  this  book  as  comprehensive  as  possible,  without  rendering  it  too  large 
for  school  purposes,  the  practical  illustrations,  under  the  general  principles,  have 
not  been  multiplied.  Besides,  it  is  not  the  best  plan  for  the  compiler  of  text-books, 
after  having  clearly  enunciated  and  demonstrated  a  general  principle,  to  go  on 
and  point  out  all  its  practical  illustrations  and  applications ;  for,  by  so  doing,  he 
deprives  the  teacher  and  student  of  the  opportunity  to  exercise  their  ingenuity  in 
performing  this  part  of  the  work.  The  instructor  should- impress  upon  the  mind 
of  the  learner  the  importance  of  forming  the  habit  of  observing  the  operations  of 
Nature  that  are  always  occurring  about  him,  and  to  exercise  his  mind  by  referring 
them  to  the  laws  or  general  principles  by  which  they  are  explained. 

The  teacher  can  make  use  of  the  Charts  in  various  ways.  He  may  use  the 
pointer  himself,  or  have  the  pupil  use  it.  A  good  plan  is  to  have  the  scholar 
exercise  himself  by  demonstrating,  with  the  pointer,  diagrams  contained  in  pre- 
vious lessons,  which  he  will  be  able  to  do  with  success  and  advantage,  having 
witnessed  the  teacher's  previous  demonstrations ;  or  the  pupil  may  do  the  same 
with  the  advance  lesson  before  listening  to  the  instructor's  explanation. 

It  is  of  the  utmost  importance  that  the  scholar  acquire  the  habit  of  oral  demon- 
stration, as  it  is  by  this  practice  that  the  teacher  is  enabled  to  know  what  the 
scholar's  difficulties  are — to  ascertain  what  he  understands,  and  what  he  does  not, 
a  quick  appreciation  of  which  is  one  of  the  greatest  secrets  of  successful  teaching. 
By  frequent  demonstrations,  the  learner,  at  the  same  time,  cultivates  the  power 
of  expressing  and  communicating  to  others  what  he  has  acquired.  Besides,  it  is 
by  demonstrating  that  definite  and  thorough  knowledge  is  obtained. 

The  pupil  should  be  frequently  exercised,  also,. in  reviewing,  in  order  to  impress 
the  idea  on  the  memory ;  for,  to  learn,  and  forget  as  readily,  is  of  but  little  use. 
.    Habitual  demonstrating  and  constant  reviewing  are  indispensable  to  tlie  highest 
degree  of  success  in  the  art  of  teaching  ;  and  these  Charts  are  particularly  adapted 
to  facilitate  both  of  these  exercises. 

Once  in  the  schoolroom,  they  are  constantly  before  the  learner,  and  at  the  com- 
mand of  the  instructor,  instead  of  being  locked  up  and  out  of  sight,  as  is  usually  the 
case  with  the  apparatus.  They  can  be  seen,  too,  at  greater  distance,  and  by 
greater  numbers  at  a  time.  Besides,  many  things  are  well  delineated  by  the  Charts 
that  could  not  be  shown  at  all  in  the  schoolroom  with  the  apparatus. 


Q  TO  THE  TEACHER 

/.;   .NEQESSIYY.ANp  USEFULNESS  OF  MAPS  AND  CHARTS. 

No  institutions  of  the  age  are  of  more  importance  than  those  of  common  educa- 
tion; of  these  none  hold  a  more  essential  rank  than  the  "  Common -Schools."  In 
these  the  first  habits  of  study  are  formed,  and  the  rudimentary  and  fundamental 
principles  of  knowledge  and  science  are  acquired  ;  and  every  experienced  teacher 
understands  the  importance  of  forming  correct  habits  of  study,  and  the  still  greater 
necessity  of  acquiring  a  lucid  and  thorough  understanding  of  the  elementary  princi- 
ples, in  order  to  comprehend  the  more  intricate  and  complicated  principles  of 
science,  that  constitute  an  endless  study  for  all  subsequent  lifetime.  Hence,  what- 
ever promotes  the  success  of  such  institutions,  or  facilitates  the  art  of  teaching, 
must  be  deserving  of  attention,  and  worthy  the  necessary  means  of  acquisition. 
When  the  scanty  supply,  the  almost  entire  absence,  of  aids  and  helps — save  a  bun- 
dle of  birch  rods  and  a  huge  oak  ruler — which  constituted  the  assistance  of  the 
teacher  a  few  years  ago,  is  contrasted  with  the  many  facilities  with  which  the 
instructor  is,  or  may  be,  surrounded  at  the  present  day,  every  one  who  has  a  child 
to  be  educated,  or  feels  himself  at  all  interested  in  the  general  education  of  his 
fellow-beings,  will  be  at  once  pleased  and  surprised  to  behold  how  great  and  rapid 
has  been  the  improvement  achieved  in  this  important  question  of  educational 
progress. 

Among  the  many  means  which,  for  the  past  few  years,  have  been  brought  to 
aid  in  this  most  essential  art  of  life,  that  of  teaching,  the  most  serviceable  and  popu- 
lar is  that  of  representing  to  the  eye  what  before  was  only  demonstrated  to  the  ear. 

How  limited  would  be  our  knowledge  of  Language,  Geometry,  Algebra,  and 
Arithmetic,  for  instance,  without  their  visible  symbols.  And  with  them  how  great 
is  our  progress  in  these  branches,  and  how  extensive  is  their  application  in  the 
development  of  other  sciences,  and  the  useful  pursuits  of  life.  Imagine  the  slow 
and  tedious  process  of  teaching  Mathematics  to  a  young  mind  through  the  ear, 
without  representing  the  same  to  the  eye  by  means  of  figures,  Utters,  and  diagrams. 
But  by  these  symbols,  with  slate  and  blackboard,  he  gives  to  these  invisible,  and, 
as  it  were,  imaginary  things,  dimensions,  localities,  and  names,  so  that  he  may  be 
able  to  see  them,  and  seeing,  grasp  them,  while  the  mind's  eye  contemplates  them 
at  leisure.  Music  could  not  rise  to  the  dignity  of  a  regular  art  until  musical  notes 
were  invented,  which  rendered  it  possible  to  express  harmonies  of  sound  to  the 
eye.  If  the  mind  may  be  so  greatly  aided  by  ocular  signs,  when  there  is  no  natural 
relation  between  them  and  the  objects  they  represent,  as  in  letters,  numerals, 
figures,  and  musical  notes,  how  much  more  must  its  power  be  increased,  when 
the  symbols  and  drawings  assume  the  pictorial  character,  and  become,  in  a  man- 
ner, actual  imitations  of  the  things  to  be  considered. 

Visible  diagrams  are  most  useful  where  definite  and  exact  properties  and  re- 
lations are  to  be  communicated  to  the  mind,  as  in  natural  science. 

Whenever  the  object  consists  of  such  fixed  elements  and  qualities   as  are 


TO  THE  TEACHER.  7 

capable  of  delineation,  but  are  not  themselves  tangible,  pictorial  illustrations  of 
some  kind  become  indispensable,  as  in  Geometry,  Trigonometry,  and  other  higher 
mathematics.  In  communicating  descriptions  of  physical  objects,  which,  in  their 
extent  and  complexity,  are  beyond  the  scope  of  direct  vision,  as  in  Geography, 
Geology,  and  Astronomy,  illustrations  and  delineations  are  indispensable.  With 
what  success  could  Geography  be  taught  or  comprehended  by  written  and  oral 
description  without  the  aid  of  maps?  A  mere  glance,  however,  of  the  eye,  at 
geographical  maps  and  astronomical  charts,  will  give  a  more  correct  appreciation 
of  the  position  and  magnitude  of  oceans,  continents,  rivers,  mountains,  states, 
counties,  and  towns,  also  of  suns,  planets,  comets,  and  stars,  than  it  were  possible 
to  obtain  by  reading  volumes  of  written  description.  A  person,  for  instance,  by 
spending  only  one  hour  in  viewing  a  well-executed  panorama  of  the  Mississippi 
river  and  its  sceneiy,  will  obtain  a  more  correct  and  lasting  impression  of  the  same, 
than  by  perusing  an  elaborate  and  well- written  description  for  weeks  and  months. 
An  entire  volume,  and  a  course  of  lectures  descriptive  of  a  large  city,  would 
fail  to  equal  one  good  cosmoramic  view,  requiring  but  jive  minute?  con- 
templation. 

It  would  seem  impossible  in  these  times  to  acquire  a  respectable  knowledge 
of  Anatomy,  Physiology,  and  Surgery,  without  the  aid  of  the  numberless  and 
elaborate  illustrations  thickly  interspersed  throughout  every  recent  book  on 
these  subjects. 

The  study  of  History,  too,  is  greatly  facilitated  by  charts  and  diagrams.  The 
study  of  Natural  History  would  be  impossible  without  the  assistance  of  pic- 
torial representations ;  but  with  the  help  of  these,  a  specimen  of  eveiy  class, 
species,  and  variety  of  man,  beast,  bird,  fish,  reptile,  and  insect,  from  every 
quarter  of  the  globe,  can  be  brought  into  the  study  or  schoolroom  to  be  con- 
templated by  the  learner  at  his  leisure. 

When  objects  to  be  considered  are  too  minute  to  admit  of  immediate  observ- 
ance, or  in  their  nature  are  wholly  invisible,  invaluable  assistance  is  rendered  by 
pictorial  descriptions,  as  in  microscopic  science  and  chemistry. 

Again,  in  the  Mechanic  Arts  and  Architecture,  how  important  is  the  aid  derived 
by  means  of  delineations.  If  it  were  attempted  to  give  a  full  description  of  a 
modern  steam-engine,  or  a  watch,  or  a  complicated  printing-press,  by  addressing 
the  imagination  and  perceptive  faculties  orally,  without  any  kind  of  illustrative 
diagrams,  volumes  would  be  required  to  impart  little  more  than  a  vague  idea 
of  these  objects ;  on  the  other  hand,  a  mere  glance  of  the  eye,  at  the  diagrams  and 
pictorial  illustrations  of  these  complicated  machines,  will  enable  even  younger 
minds  to  obtain  quite  a  clear  understanding  of  their  construction  and  operation. 

Moral  sentiments,  too,  cannot  possibly  by  any  other  method  be  half  so  immov- 
ably impressed  upon  the  mind  as  by  means  of  pictures.  Henry  Ward  Beecher 
says  he  used  to  look  at  the  picture  of  the  "  boy  stealing  apples  "  till  he  fairly  wore 
out  that  leaf  of  his  spelling-book.  All  children  will  read  "  ^Esop's  Fables  "  with 
the  pictures,  while  hardly  any  will  read  them  without  the  illustrations.  Grown 


g  TO   THE  TEACHER. 

pei-sons  as  well  as  children  will  have  their  attention  excited  when  real  objects 
are  brought  before  them,  and  in  the  absence  of  real  objects,  good  pictures  will 
interest  them  equally ;  and  this  is  for  the  reason  that  their  understanding  is  so 
much  aided  by  them  as  to  receive  gratification,  and  yield  the  ready  pleasure  of 
KNOWING.  But  in  oral  and  written  descriptions  alone,  the  mental  effort  required 
to  compreJiend  overpowers  curiosity,  which  renders  it  a  task  to  give  attention,  and 
thus  destroys  the  pleasure  of  study.  In  conversation  on  common  subjects,  even, 
do  we  not  almost  instinctively  catch  up  pen  or  pencil,  and  represent  to  the  eye 
what  we  are  attempting  to  describe  to  the  ear  ?  And  this  is  because  we  can  make 
ourselves  so  much  more  easily  and  distinctly  apprehended. 

Books,  which  but  few  years  ago  were  made  in  solid  pages  of  reading  matter, 
presenting  a  dull,  monotonous  appearance,  are  now  relieved  and  enlivened  by 
a  thousand  interesting  illustrations,  each  page  inviting  the  learner  and  claiming 
his  attention  with  some  lucid  and  instructive  picture.  Many  subjects  which 
before  were  considered  dry  and  uninteresting,  and  even  beyond  the  capacity  of 
ordinary  minds,  and  therefore  seldom  pursued  by  them,  or,  if  so,  with  an  obscure- 
ness  that  amounted  to  a  waste  of  time,  have  now  come  to  be  so  clearly  explained, 
by  means  of  this  picture-making  art,  as  to  be  brought  within  the  comprehension 
of  the  most  ordinary  minds ;  thus  causing  the  natural  sciences,  especially,  to  be 
more  generally  read  and  appreciated  than  were  possible  by  any  other  method. 

The  superiority  of  the  eye  over  all  other  senses,  as  a  means  of  education,  is 
undeniable,  for  it  has  been  demonstrated  beyond  a  question.  No  other  system  of 
teaching  renders  the  acquisition  of  knowledge — especially  scientific  knowledge — 
so  pleasant  and  agreeable  to  the  learner.  With  appropriate  diagrams  and  draw- 
ings, properly  demonstrated,  scholars  will  become  fascinated  in  studying  those 
principles  and  sciences  which  before  they  dreaded,  and  pronounced  tedious  and 
irksome,  and  tried  their  utmost  to  avoid.  This  method  not  only  makes  study  a 
pleasure,  both  to  teacher  and  pupil,  but  it  greatly  economizes  their  time  and 
labor,  and  produces  in  the  learner  the  habit  of  demonstrating  ;  giving  the  acquired 
knowledge  of  general  principles  an  exactness  and  fixedness  in  the  mind  which 
enables  him,  in  after-life,  to  make  a  ready  and  practical  application  of  his 
education. 

In  short,  Authors,  Lecturers,  and  Teachers,  in  almost  every  branch  of  learning, 
have  found,  by  the  infallible  test  of  experience,  that  they  may  POUR  knowledge, 
in  large  measures,  through  the  eye,  and  impress  it  indelibly  on  the  mind.  Other- 
wise, why  have  books  on  all  subjects  come  to  be  so  filled  up  with  pictures? 
There  is  hardly  a  work  now,  on  the  natural  sciences,  at  least,  but  what  contains 
hundreds  of  illustrations.  Pictures  are  no  longer  made  solely  for  the  amusement 
of  children.  Books  treating  of  the  most  intricate  sciences,  even,  have  come  to  be, 
emphatically,  " Picture-Books" 

Already  many  books  may  be  read  by  their  pictures.  Less  reading  matter  and 
more  pictures  is  fast  becoming  the  motto  in  book-making. 

Of  the  various  methods  of  pictorial  illustrations,  that  of  maps  and  charts,  on  a 


10  THE  TEACHER.  9 

large  scale,  for  the  use  of  teachers  and  lecturers,  is  the  most  serviceable,  as  it  en- 
ables the  instructor  to  make  his  demonstrations  to  a  whole  class,  school,  or  public 
audience  of  thousands,  even,  with  the  same  time  and  effort  required  to  give  the 
same  explanation  to  one  learner  alone.  Especially  useful  and  necessary  are  such 
maps  and  charts  in  common-schools  and  academies,  where  the  learners  are  begin- 
ners, and  consequently  their  powers  of  abstraction  as  yet  undeveloped.  Even  the 
"  A,  B,  C's"  and  "  a-b  ab's,"  addition  and  multiplication  tables,  etc.,  are  now  exten- 
sively printed  in  the  form  of  charts,  and  at  once  placed  before  the  whole  school  or 
class,  and  are  thus  taught  with  a  hundredfold  greater  success  than  by  the  old  method 
of  calling  up  one  youngster  at  a  time  and  pointing  with  a  pin  at  a  dozen  small, 
obscure,  half-obliterated  letters,  and  telling  him  "  that  is  A,  tliat  is  B,  that  is  C,"  etc., 
then,  shutting  the  book,  sending  him  to  fold  his  hands  for  the  next  two  or  three 
hours,  to  gaze  at  nothing  but  the  blank  walls.  If  the  assistance  rendered  by  charts 
be  so  great  in  teaching  these  mere  symbols  and  simplest  rudiments,  how  much 
greater  must  be  their  utility  in  teaching  those  general  principles  which  constitute 
the  basis  of  several  important  sciences,  as  in  the  study  of  Natural  Philosophy. 

Natural  Philosophy,  as  treated  in  schoolbooks,  being  composed  of  a  description 
of  foe  fundamental  and  leading  principles  of  several  sciences,  as  Mechanics,  Acous- 
tics, Optics,  Electricity,  Astronomy,  etc.,  becomes,  therefore,  the  common  branch 
of  education  most  generally  pursued  by  scholars  after  acquiring  a  degree  of  pro- 
ficiency in  the  more  purely  rudimentary  branches.  Natural  Philosophy,  too,  being 
of  an  abstract  nature,  especially  to  beginners  and  young  minds,  it  is  of  the  greatest 
necessity  that  its  leading  principles  be  represented  to  the  eye  of  the  learner  in  the 
most  lucid  and  simple  form  possible,  that  he  may  receive  a  clear,  strong,  and  lasting 
impression  of  the  important  principles  that  make  up  this  most  essential  branch  of 
common  education.  Natural  Philosophy  is  so  almost  universally  applicable,  in 
one  or  another  form,  to  the  useful  pursuits  of  life,  that  every  scholar  should  pursue 
it  and  engraft  its  principles  deep  in  his  mind,  before  he  set  out  on  his  practical  life 
in  earnest.  If  any  one  study  is  to  be  more  thoroughly  demonstrated  and  mastered 
than  another,  it  should  be  this.  But  it  is  the  opinion  of  all  teachers,  that  without 
the  aid  of  appropriate  apparatus  or  drawings.  Natural  Philosophy,  especially,  can 
be  taught  with  but  little  success.  To  draw  the  necessary  diagrams  on  the  black- 
board, from  day  to  day,  requires  too  much  of  the  teacher's  time,  and  so  much  of 
his  patience  and  skill  that  he  seldom  draws  them  at  all ;  and,  if  he  draw  them,  they 
are  erased  from  the  board  the  next  half-hour :  and  to  obtain  the  real,  necessary 
philosophical  apparatus,  is  too  expensive  to  be  generally  afforded.  Of  common- 
schools  and  academies,  hardly  one  in  a  thousand  can  afford  the  apparatus.  Many 
thousand  dollars  would  be  required  to  purchase  all  the  apparatus  represented  by 
these  charts ;  yet  these  will  serve  all  the  general  purposes  of  a  complete  set  of  ap- 
paratus, and  in  some  respects  answer  better,  and  are  not  so  expensive  but  that 
every  school  may  obtain  them. 

BROOKLYN,  N.  Y.,  January  1,  1872. 


COISTTEJ^TS. 


INTRODUCTION. 

MATTER,  FORCE,   MOTION,  AND  MECHANICS. 

CHAPTER  I. 

(CHART  NO.   1.) 

[References  are  to  paragraphs,  not  to  pages.] 

Definitions  and  Preliminary  Principles— Matter— Different  kinds  of  matter- 
Simple  elements,  1 — Changes  in  matter,  chemical  or  physical,  2 — Imponderables : 
light,  heat,  and  electricity,  3 — Atoms,  or  ultimate  constitution  of  matter,  4 — Mole- 
cules, 5 — The  properties  of  matter  are  general  or  specific,  6 — Physical  and  chemical 
properties  of  matter,  7— Physics,  or  Natural  Philosophy,  and  Chemistry,  8. 

CHAPTER   II. 

Definitions  and  General  Properties  of  Matter— ESSENTIAL  PROPERTIES  OF  MATTER  : 
magnitude  or  extension,  9 — Impenetrability,  10 — SECONDARY  OR  ACCESSORY  PROPERTIES 
OF  MATTER:  divisibility,  11 — Compressibility,  12 — Expansibility,  13 — Porosity — Phy- 
sical pores — Sensible  pores,  14 — Mobility — Motion  and  rest  are  relative  and  absolute, 
15 — Inertia — Inertia  and  momentum  (or  motion),  equally  inherent  conditions  of 
matter,  16— Indestructibility,  17— ATTRACTION  ;  Attraction  of  gravitation— Terres- 
trial attraction— Cohesion— Adhesion— Affinity,  18 — VARIETIES  OF  MOTION:  Transla- 
tion and  direct  motion,  19 — FORCES,  20 — Momentum,  21 — Cohesion  and  repulsion,  22 
— Relation  of  cohesion  and  repulsion  in  the  three  states  of  matter,  23 — Structure  in 
solids — The  formative  force  of  matter — Crystalline  and  organic  forms,  24. 

The  Characteristic  Properties  of  Solids— Crystalline  form,  25— ELASTICITY  :  Ten- 
sion— Flexure — Torsion,  26 — Resistance  to  fracture,  27 — Hardness,  28 — Malleability, 
29— Ductility,  30— Flexibility  and  pliability,  31— Brittleness,  32— Hardening  and 
annealing,  33— Welding,  34. 

CHAPTER   III. 

ATTRACTION. 

Molecular  Attraction— Fig.  1,  Interstices  between  atoms  and  molecule?  of  matter, 
35— Fig.  2,  Cohesive  attraction,  36— Fig.  3,  Adhesive  attraction,  37— Fig.  4,  Phe- 
nomena of  capillarity,  38. 

Gravitation — Weight,  39— Fig.  5,  Centre  of  gravity  of  bodies— Equilibrium  of  bodies, 
stable,  unstable,  and  neutral,  40— Fig.  6,  Method  of  finding  the  centre  of  gravity, 
41— Fig.  7,  Neutral  and  unstable  equilibrium,  42— Fig.  8,  Stability  of  bodies,  43— 
Fig.  9,  Relative  stability  of  cubes  and  pyramids,  44— Fig,  10,  Centre  of  gravity  of 
vehicles,  45— Fig.  11,  Centre  of  gravity  in  man,  46— Fig.  12,  Law  of  intensity  of 
gravity— Tabular  statement  of  the  law,  47. 


12  CONTENTS. 

Conditions  affecting  Gravity — Gravity  affected  by  altitude,  48— Gravity  affected 
by  depression  below  level  of  the  sea,  49— Gravity  affected  by  shape  of  the  earth,  50 

Gravity  affected  by  the  earth's  rotation,  51 — Earth  drawn  toward  falling  bodies, 

52 Direction  of  gravity — Up  and  down,  relative  terms,  53. 

CHAPTER  IV. 

Motion  and  Force — MOTION  :  Variety  of  motions — Uniform,  accelerated,  and  re- 
tarded motion — FORCES  :  Definition  of  force — Unit  of  force — Direction  and  intensity 
of  forces — Equilibrium  of  forces — Measure  of  force  stated  in  four  propositions,  54 — 
Pig1.  13,  Laws  of  falling  and  rising  bodies — Tabular  statement — Finding  the  veloci- 
ty of  rising  and  falling  bodies,  55 — Fig.  14,  Accelerated  velocity  illustrated  by  flow 
of  liquids,  56 — Fig".  15,  Keflected  motion,  57 — Fig".  16,  Eesultant  motion — Com- 
pound and  resultant  forces — Parallelogram  of  velocities,  motions,  and  forces — Com- 
position and  resolution  of  forces,  58 — Fig-.  17,  Action  of  wind  on  sails  of  vessels, 
59 — Fig-.  18,  Compensating  pendulum,  60 — Fig.  19,  Laws  of  oscillation  of  the  pen- 
dulum— General  propositions — Scientific  uses  of  the  pendulum,  61 — Fig-.  20,  Motion 
of  projectiles,  vertically  upward,  downward,  and  in  other  directions — Greatest  hori- 
zontal range  of  projectiles,  62— Fig-.  21,  Perpetual  revolution,  63— Fig-.  22,  Falling 
of  projectiles  thrown  from  horizontal  guns,  64 — Fig1.  23,  Action  and  reaction  are 
equal,  65. 

CHAPTER  V. 

MECHANICAL  POWERS. 

Levers — Definition  of  machine,  motor,  power,  weight,  etc. — Equilibrium  of  force  and 
resistance — Fig-.  24,  Lever  of  the  first  class — Conditions  of  equilibrium  with  all 
levers — Formulae  for  finding  the  power,  weight,  and  arms — Example,  66 — Fig.  25, 
Lever  of  the  second  class — Example,  67 — Fig-.  26,  Lever  of  the  third  class — Example, 
68— Fig-.  27,  Compound  lever— Formulae— Example— Samples  of  the  different  levers, 
69— Fig.  28,  Limbs  of  animals  levers  of  the  third  class,  70. 

Wheel  and  Axle — Fig.  29,  The  wheel  and  axle— Formulae  for  calculating  the 
different  parts — Example,  71 — Fig-.  30,  Simple  windlass  a  modification  of  wheel 
and  axle,  72 — Fig.  31,  Chinese  differential  windlass,  73 — Fig.  32,  Compound  wheel 
and  axle — Formulas  for  calculating  the  parts — Example,  74. 

Pulleys — Fig-.  33,  Simple  fixed  pulley — The  law  of  the  pulley — Use  of  the  simple 
pulley — Example,  75 — Fig-.  34,  Simple  movable  pulley — Formulae — Example,  76 — 
Fig-.  35,  Movable  and  immovable  pulley — Formulae — Example,  77 — Fig.  36,  A 
system  of  pulleys  with  more  than  one  cord — Formulae — Example,  78 — Fig.  37,  Com- 
pound pulleys  with  two  or  more  movable  pulleys — Formulae — Example,  79 — 
Fig.  38,  Compound  pulleys  with  one  movable  pulley — Formulae — Example,  80 — 
Fig.  39,  A  system  of  pulleys  with  more  than  one  rope  and  three  cords  to  each  pul- 
ley— Formulae — Example,  81 — Fig.  40,  A  system  of  pulleys  with  two  ropes,  having 
one  fixed  and  two  movable  pulleys — Example,  82. 

Inclined  Plane — Fig.  41,  Inclined  plane — Conditions  of  equilibrium — Formulae — 
Example,  83— Fig.  42,  The  screw  a  modification  of  the  inclined  plane— Formulae- 
Example,  84 — Fig.  43,  The  wedge  a  modification  of  the  inclined  plane — Conditions 
of  equilibrium — Formulae,  85 — Fig.  44,  Endless  screw — Combination  of  the  five 
mechanical  powers — Formulae — Example,  86. 

CHAPTER  VI. 
(CHART  NO.  2.) 
HYDROSTATICS. 

Distinguishing  properties  of  Solids,  Fluids,  and  Gases— Attraction  and  re- 
pulsion— Rigidity  of  bodies  caused  by  preponderance  of  cohesion — Fluidity  caused 


CONTENTS.  13 

by  equilibrium  between  cohesion  and  repulsion — Gaseous  condition,  caused  by  pre- 
ponderance of  repulsion — Definition,  87 — Mobility  of  liquids — Cause  of  different 
degrees  of  mobility — Heat  increases  mobility — Ultimate  atoms  of  gaseous  bodies  as 
bard  as  those  of  solids,  88 — Compressibility  of  liquids,  89 — Cohesion  in  liquids,  90 — 
Repulsion  in  gases,  91. 

Pressure  of  Liquids — Figr.  1,  Liquids  transmit  pressure  equally  in  all  directions 
— Pressure  at  every  point  perpendicular  to  the  surface,  92 — Fig.  2,  Pressure  of 
liquid  not  in  proportion  to  its  quantity  but  to  its  height,  93 — Fig.  3,  Equilibrium 
of  liquids  in  communicating  vessels,  94 — -Artesian  wells,  95 — Fig-.  4,  The  water- 
level,  96— Fig-.  5,  The  spirit-level,  97— Fig-.  6,  Tendency  of  liquids  to  seek  a  level 
shown  by  aqueducts,  98 — Fig-.  7,  Intermitting  springs,  99 — Fig-.  8,  Upward  pressure 
of  liquids  equal  to  downward  pressure  at  the  same  depth — Buoyant  effort  of  fluids, 
100 — Fig.  9,  Downward  pressure  of  liquids  independent  of  shape  and  capacity  of 
containing  vessel,  101 — Equilibrium  of  liquids  of  different  densities,  102 — Fig.  1O, 
Pressure  of  a  liquid  is  in  proportion  to  its  height  and  the  area  of  its  base,  103 
— Fig.  11,  Pressure  of  liquids  on  the  sides  of  a  vessel,  104 — The  total  pressure  upon 
the  walls  of  a  vessel,  105 — The  total  pressure  on  the  bottom  and  sides  of  a  vessel, 
106 — Fig.  12,  Hydrostatic  paradox,  107 — Fig.  13,  Practical  use  of  the  principle 
that  liquids  transmit  pressure  in  all  directions  alike — Formulae,  108 — Fig.  14,  Hy- 
drostatic press — Formulae — Example,  109 — Fig.  15,  Bursting  a  cask  with  hydrostat- 
ic pressure,  110 — Fig.  16,  Hydrostatic  bellows,  111 — Fig.  17,  Hydrostatic  pressure 
in  mountains,  112 — Fig.  18,  Submerged  bodies  not  pressed  in  all  directions  equally 
— Upward  pressure  the  buoyant  effort  of  the  fluid,  113. 

Specific  Gravity — Specific  gravity,  what  it  is,  and  how  found,  114 — Fig.  19, 
Method  of  finding  specific  gravity  of  solids — Rule  for  solids  heavier  than  water — 
Rule  for  solids  lighter  than  water— Rule  for  liquids— Examples,  115— Fig.  20, 
Specific  gravity  of  liquids,  continued — Hydrometer  —  Example,  116  —  Fig.  21, 
Liquids  of  unequal  densities  seek  different  levels  in  the  same  vessel,  117 — Fig.  22, 
Principles  of  flotation — The  plane  of  flotation — A  toy  illustrating  flotation,  118. 

CHAPTER   VII. 
(CHART  NO.  2.) 
PNEUMATICS. 

General  Principles — Definitions — Gases — Vapors — Tension,  119 — Gases  are  simple 
or  compound — Expansion,  120 — Mechanical  condition  of  gases,  121. 

Atmospheric  Air — Fig.  23,  The  atmospheric  air  an  aerial  ocean  enveloping  the 
earth— Inequalities  of  the  earth's  surface,  122— Height  of  the  atmosphere — Its 
weight  and  elasticity  in  equilibrium,  123— Composition  of  the  atmosphere— Its 
adaptation  to  animal  life  and  combustion— Compensatory  relation  of  animal  res- 
piration and  vegetation,  124— Fig.  24,  Impenetrability  of  gases,  125— Fig.  25, 
Pressure  or  weight  of  the  atmosphere — Its  pressure  equal  in  all  directions — Amount 
of  its  pressure  on  the  human  body— All  physical  pores  filled  with  it,  126— Fig.  26, 
Compression  and  expansion  of  the  atmosphere,  127— Fig.  27,  Air-pump,  receiver, 
and  vacuum,  128— (Fig.  27),  Various  phenomena  in  vacuo :  no  combustion ;  no  flight  ; 
no  life ;  no  "  suction ;"  no  resistance,  129— Fig.  28,  Pressure  of  air  equal  in  all 
directions,  shown  by  hollow  hemispheres,  130— Fig.  29,  Expansion  fountain,  131— 
Fig.  30,  Atmospheric  pressure  variable  at  the  same  place— Equilibrium  of  hydro- 
static and  atmospheric  pressures,  132  —  Atmospheric  pressure  sustains  different 
liquids  at  different  heights  :  their  heights  being  inversely  as  their  specific  gravities 
— Example — "Water  and  mercury,  133. 

The  Barometer— Fig.  31,  The  Barometer  and  its  uses,  134— Height  of  the  mercury 
at  different  elevations,  135— Barometer  as  a  weather-glass— Rules  for  reading  the 


14  CONTENTS. 

changes  of  the  barometer,  136— Diurnal  variations  of  the  barometer,  137— Pig.  32, 
The  wheel-barometer,  138 — Pig1.  33,  Density  of  the  atmosphere  at  different  alti- 
tudes, 139— Fig-.  34,  Balloons,  140— Pig1.  35,  Diving-bells,  141— Fig.  36,  Atmos- 
pheric pressure  shown  by  inverted  tumbler  of  water,  142 — Fig.  37,  Atmospheric 
pressure  shown  by  currents  of  air,  143 — Fig.  38,  Atmospheric  pressure  shown  by 
tubes  and  water,  144 — Fig.  39,  Vacuum  fountain,  illustrating  atmospheric  pressure, 
145 — Fig.  40,  Animal  respiration  dependent  upon  atmospheric  pressure,  146 — Fig. 
41,  Mariotte's  Law  relating  to  the  elastic  force  of  gases,  147 — Fig-.  42,  Condenser  and 
condensed  air,  148— Fig.  43,  Condensed  air  fountain,  149— Fig.  44,  Air-gun,  150. 

CHAPTER   VIII. 

(CHART  NO.  3.) 

HYDRAULICS. 

General  Principles — Definitions,  151— Shape  of  orifices,  152— Friction  between 
liquids  and  solids,  153— Fig.  1,  Velocity  and  gravity— Projected  streams  subject 
to  the  same  laws  as  other  projectiles,  154 — Fig.  2,  Velocity  of  discharge — Tabular 
statement,  155— Fig.  3,  Flowing  of  rivers,  156— Fig.  4,  Finding  the  velocity  of 
rivers,  157. 

Water  as  Motive  Power — Water  as  motive  power,  158 — Fig-.  5,  Overshot  water- 
wheel,  159— Fig-.  6,  Breast  water-wheel,  160— Fig-.  7,  Undershot  water-wheel,  161 
— Fig.  8,  Turbine  water-wheel,  162 — Fig.  9,  Keaction  and  centrifugal  machine,  or 

*  Barker's  mill,  163. 

Machines  for  Elevating  Water— Variety  of  water  elevators,  164— Fig-.  10,  Lift- 
ing wheel,  165— Fig.  11,  Wheel  and  buckets,  or  Persian  wheel,  166— Fig:.  12,  Endless 
chain  of  pots,  167— Fig.  13,  Chain-pump,  168— Fig1.  14,  First  invented  centrifugal 
pump,  169— Fig-.  15,  The  T-centrifugal  pump,  170— Fig.  16,  Archimedes'  screw, 
171— Fig-.  17,  Hydraulic  ram,  172. 

Suction  Pumps— Fig-.  18,  The  principle  of  suction  pumps,  173— Fig-.  19,  Proof  of 
atmospheric  pressure  in  pumps,  174 — Fig.  20,  Double  cylinder  rotary  pump,  175 — 
Fig.  21,  Single  cylinder  rotary  pump,  176 — Fig-.  22,  Double  cog-wheel  rotary 
pump,  177 — Fig.  23,  Bellows  suction  pump,  178 — Fig-.  24,  Diaphragm  suction  pump, 
179_Fig.  25,  Plunger  suction  and  force  pump,  180— Fig-.  26,  Single  cylinder  suc- 
tion pump,  181 — Fig.  27,  Suction  and  force  pump,  182 — Fig-.  28,  Double  acting  suc- 
tion and  force  pump,  183 — Fig.  29,  Single-acting  suction  and  force  pump,  184  — 
Fig.  30,  Double-acting  suction  and  force  pump,  with  only  two  valves,  185— Fig. 
31,  Fire-engine,  or  two  cylinder  force  pump,  186— Fig.  32,  Stomach  pump,  187. 
Syphons,  Fountains,  etc — Fig.  33,  The  syphon  dependent  on  atmospheric  pres- 
sure, 188— Intermittent  springs,  189— Fig.  34,  Sharp  angles  obstruct  flow  of  liquids 
— Shown  by  syphons,  190 — Fig.  35,  Conveying  water  over  hills  with  syphons,  191 
—Fig.  36,  Syphon  for  the  chemical  laboratory,  192— Fig.  37.  Loss  of  effective  head 
in  public  water- works,  193 — Lateral  pressure  of  liquids  diminished  by  their  motion, 
194— Fountains,  and  vertical  jets  of  water,  195— Fig.  38,  Hiero's  fountain,  196— 
Fig.  39,  Intermittent  fountains— Importance  of  water— Importance  of  hydraulic  and 
hydro-pneumatic  machines,  197. 

CHAPTER    IX. 

(CHART  NO.  4) 
HEAT  AND  STEAM-ENGINE. 

Preliminary  General  Principles  Relating  to  Heat— Definitions,  198— Heat  and 
cold  relative  terms,  199— Temperature,  200  —  Nature  of  heat— Emission  theory— 
Undulatory  theory,  201 — GENERAL  EFFECTS  OF  HEAT  ;  it  expands  bodies  ;  is  a  source 


CONTENTS.  15 

of  mechanical  energy ;  it  determines  the  distribution  of  animals  and  plants ;  it  more 
or  less  controls  chemical  affinity,  and  limits  vital  forces,  202 — Equilibrium  and 
transference  of  heat,  203 — Luminous  and  obscure  heat,  204. 

Sources  of  Heat— PHYSICAL  SOURCES  OF  HEAT:  Solar  radiation,  205— Quantity  of  heat 
emitted  by  the  sun,  206 — Extremes  of  natural  temperature,  207 — Terrestrial  radi- 
ation, 208 — Atmospheric  electricity,  209 — CHEMICAL  SOURCES  OP  HEAT,  210 — Combus- 
tion, 211 — MECHANICAL  SOURCES  OF  HEAT;  Friction — Compression — Percussion,  212 — 
PHYSIOLOGICAL  SOURCES  OF  HEAT:  Animal  and  vegetable,  213 — Difference  between 
quantity  and  intensity  of  heat,  214. 

EXPANSION. 

Solids — Fig.  1,  Linear  expansion  of  solids — Pyrometers — Laws  of  expansion,  215 — 
Coefficient  of  expansion,  lineal  and  cubic,  216 — Fig.  2,  Cubic  expansion  of  solids, 
217 — Relation  between  linear  and  cubic  expansion,  218 — Amount  of  expansion  of 
solids  absolute  and  relative — Table  of  expansion  of  solids,  219. 

Liquids— Fig:.  3,  Expansion  of  liquids,  220— The  amount  of  expansion  of  liquids, 
221 — Fig-.  3,  Different  liquids  expand  differently  for  the  same  increase  of  temper- 
ature—Table of  expansion  of  liquids,  222— Water  at  certain  temperatures  an  ex- 
ception to  the  laws  of  contraction  and  expansion,  223 — Beneficial  effects  of  unequal 
expansion  of  water,  224 — Freezing  of  water  in  small  tubes,  225. 

Gases— Fig.  4,  Expansion  of  gases,  226— The  general  laws  of  expansion  of  gases  by 
heat,  227 — Relation  between  compressibility  and  expansibility,  228 — Density  of 
gases,  229. 

Specific  Heat— Fig.  5,  Calorimetry,  230— Specific  heat  or  caloric  capacity,  231— 
Unit  of  heat,  or  thermal  unit,  232 — Standard  of  specific  heat — Table  of  specific  heat 
of  different  substances,  233 — Effects  of  specific  heat  of  water  on  climate,  234 — Spe- 
cific heat  of  gases,  235 — Fig.  6,  Compression  of  air  and  other  gases  diminishes  their 
capacity  for  heat,  236 — Tivo-sevenths  of  the  heat  applied  to  warming  houses  is  con- 
sumed in  expanding  the  air,  237 — Specific  heat  affected  by  change  of  state — Table 
of  specific  heat  of  different  states  of  bodies,  238. 

COMMUNICATION   OP   HEAT. 

Heat  is  communicated  by  conduction,  convection,  and  radiation,  229. 

Conduction  of  Heat — SOLIDS  :  Conduction  of  heat — Conductors  and  non-conductors, 
240 — Different  solids  conduct  heat  differently,  241 — Fig.  7,  Determination  of  the 
conductibility  of  solids — Table  of  conductibility  of  solids,  242 — Musical  tones  caused 
by  conduction,  243 — Conductibility  varies  with  molecular  arrangement,  244 — Fig.  8, 
Conduction  the  principle  of  the  safety-lamp  of  Davy,  245 — LIQUIDS  :  Conductibility 
of  liquids,  246 — Fig.  9,  Heat  in  liquids  not  equalized  by  conduction,  247 — Fig.  10, 
Non-conductibility  of  liquids  shown  by  experiments  with  water  and  ice,  248— GASES  : 
Conductibility  of  gases,  249— Relative  conductibility  of  moist  and  dry  air,  250— Re- 
lative conductibility  of  solids,  liquids,  and  gases  of  the  same  temperature,  251— The 
philosophy  of  clothing,  252. 

Convection  of  Heat— LIQUIDS  ;  Convection  of  liquids,  253— Ocean  currents— The 
Gulf  Stream,  254 — Fig.  11,  Heating  buildings  by  convection  of  fluids  in  pipes,  255 
—GASES  :  Convection  of  Gases,  256— Heating  buildings  by  steam,  257— The  atmo- 
sphere an  immense  steam  heating  apparatus,  258 — Relation  of  air  to  the  earth  same 
as  glass  to  a  hot-house,  259. 

Winds— Definition,  260— KIXDS  OF  WIND  :  Regular,  Variable,  Periodical,  Hurricanes, 
Tornadoes,  261 — Fig.  12,  Cause  of  wind — Trade  winds,  262 — Variable  winds,  263 — 
Land  and  sea-breezes,  264— Hurricanes  or  cyclones,  265— Tornadoes  or  whirlwinds 
—Water-spouts,  266— Physical  properties  of  winds  :  hot,  dry,  moist,  etc.,  267— Table 
of  general  direction  or  frequency  of  different  winds,  268— Fig.  13,  Anemometers- 
Pressure  of  winds,  269 — Velocity  of  winds — Table  of  velocity  and  force  of  wind,  270. 


16  CONTENTS. 

Measure  of  Temperature—  Thermometers,  271— Fig:.  14,  Mercurial  thermometers : 
Fahrenheit,  Centigrade,  Reaumur,  272— Rule  of  conversion  of  thermometric  scales 

Example,  273 — Fig-.  15,  Method  of  making  a  thermometer,  274 — Standard  points 

of  the  thermometer,  275 — Fig-.  16,  Method  of  graduating  thermometers — Fixing  the 
freezing  point,  276 — Fig-.  17,  Fixing  the  boiling  point  of  thermometers,  277 — Tests 
of  thermometers,  278 — Sensibility  of  a  thermometer,  279 — Limits  of  the  mercu- 
rial thermometer — Pyrometers,  280 — Spirit  thermometers — Air  thermometers,  28]  — 
Fig-.  18,  Self-registering  thermometers,  282 — Fig.  19,  Differential  thermometers,  283. 

Radiation  of  Heat — Radiation  of  heat,  284 — Cooling  by  radiation,  285 — Intensity 
of  radiation — Laws  of  radiation,  286 — Radiant  heat  is  partially  absorbed  by  the 
medium  through  which  it  passes,  287 — Radiation  in  vacuo,  288 — Universal  radia- 
tion and  constant  mutual  exchange  of  heat  between  bodies,  289. 

ACTION   OP   DIFFERENT   BODIES   UPON   HEAT. 

Surface  Action — Incident  heat  absorbed  and  reflected,  290 — Fig.  20,  Laws  which  gov- 
ern reflection  of  heat — Angles  and  planes  of  incidence  and  reflection,  291 — Fig.  21, 
Reflection  of  heat  from  concave  mirrors,  292 — Reflective  power  of  different  sub- 
stances, 293 — Determination  of  reflective  power,  294 — Fig.  22,  Absorptive  power — 
Relative  absorptive  power  of  different  substances — Absorptive  power  of  colors,  295 
— Emission  or  radiating  power  of  different  substances,  296 — Causes  which  modify 
the  reflective,  absorbent,  and  emission  powers  of  bodies;  as  polish,  density,  direction 
of  rays,  source  of  heat  and  color,  297. 

Diathermancy — Refraction — Polarization — Transmission  of  radiant  heat,  298 — 
Causes  which  modify  the  diathermanic  power  of  bodies,  299 — Diathermancy  of  the 
air,  300— Fig.  23,  Refraction  of  heat,  301— Polarization  of  heat,  302. 

CHANGE  OF  STATE  OF  BODIES  BY  THE  ACTION  OF  HEAT. 

Latent  Heat— laquef action  and  Solidification— Latent  heat  of  fusion,  303— Li- 
quefaction and  solidification,  or  melting  and  freezing — The  laws  of  liquefaction  and 
solidification — Table  of  melting  points  of  different  substances — Decomposition  by 
heat — Refractory  bodies,  304 — Peculiarities  in  the  fusion  of  certain  solids,  305 — 
Melting  and  freezing  always  gradual — Melting  is  a  cooling,  and  freezing,  a  warm- 
ing process,  306 — Why  ice  does  not  acquire  great  thickness,  307 — Latent  heat,  irreg- 
ular expansion,  and  high  specific  heat  of  water  graduate  the  changes  of  the  seasons, 
308 — Freezing  mixtures,  309 — Crystallization,  310. 

Vaporization — Definitions — Vaporization,  311 — Volatile  liquids  and  fixed  liquids, 
312 — Latent  heat  of  evaporation,  313 — Latent  heat  of  steam,  314 — Latent  and  sen- 
sible heat  of  steam  at  different  temperatures,  315. 

Ebullition  or  Boiling— Fig-.  24,  Ebullition,  316— Laws  that  govern  the  phenomena 
of  ebullition,  317 — Causes  that  modify  the  boiling  point  of  liquids — Fig.  25,  Variation 
of  pressure  varies  the  boiling  point — Water  boiled  by  application  of  cold,  318 — Use- 
ful applications  of  boiling  water  under  diminished  pressure — Boiling  point  affected 
by  altitude — Fig.  26,  Franklin's  pulse-glass,  319 — Boiling  point  affected  by  solution 
of  solids  in  the  liquid,  320 — Nature  of  the  vessel  varies  the  boiling  point,  321. 

Evaporation — Evaporation,  322 — Fig.  27,  Evaporation  in  a  vacuum — Laws  of  evap- 
oration—Limit of  tension,  323— Fig-.  28,  Evaporation  under  pressure,  324— Heat  in- 
creases and  cold  decreases  the  tension  of  vapors,  325 — Causes  that  accelerate  evap- 
oration are  pressure,  increase  of  surface,  dryness  of  air,  and  circulation  of  air,  326. 

Condensation — Causes  of  condensation  are  chemical  action,  pressure,  and  diminu- 
tion of  temperature,  327 — Dew-point,  328. 

Steam. — Fig.  29,  Pressure  exerted  by  steam  or  heated  vapor,  329 — Fig.  30,  Candle- 
bombs, illustrating  the  explosion  of  steam-boilers,  330 — Spheroidal  state  of  liquids 
— Causes  of  the  spheroidal  state  of  liquids,  331 — Fig.  31,  Condensation  of  steam,  332 
— Fig.  32,  Illustration  of  the  principle  of  the  low-pressure  engine,  333 — Fig.  33, 


CONTENTS.  17 

High-pressure  steam — Boiling  point  under  high  pressure — Table  of  boiling  point  of 
water  at  different  atmospheric  pressures,  334. 

Frost-Bearer — Bain,  Snow,  etc.  —  Fig.  34,  Freezing  by  evaporation— The  Cry- 
ophorus,or  frost-bearer,  335— Rain,  336— Snow,  337— Hail,  338— Fig-.  35,  Bain-gauge, 
;>39 — Distribution  of  rain,  340 — Days  of  rain — Table  of  rainy  days  in  different  lati- 
tudes, 341 — Annual  depth  of  rain  in  different  places,  342 — Fig-.  36,  Hygrometer,  or 
moisture-measurer — Dew-point  varies  with  the  moisture  in  the  air,  343. 

Combustion — Fig-.  37,  Combustion  and  structure  of  flame — Elements  of  organic 
bodies — Elements  of  combustibility — Carbon  and  hydrogen  burn  differently — Hy- 
dro-carbons the  best  illuminators,  344. 

Steam-Engines—Origin  of  the  steam-engine,  345— Fig-.  38,  The  Eolipile,  346— Im- 
provements in  steam-engines,  347 — Reciprocating  and  rotary  motions  of  engines, 
348— Fig-.  39,  The  high-pressure  engine,  349— Fig-.  40,  The  eccentric— Its  impor- 
tance, 350— Fig-.  41,  Steam-boiler,  and  operation  of  steam-valves,  351— Condensa- 
tion in  steam-engines,  352— Fig-.  42,  Stuffing-boxes,  353— Fig-.  43,  The  low-pressure 
or  condensing  engine— Its  operation— The  governor— The  fly-wheel,  etc.,  354  (see 
frontispiece). 

CHAPTER    X. 

(CHART  NO.  5.) 

OPTICS. 

General  Properties  of  Light— Optics— Light,  355— Nature  of  Light — Theories: 
Corpuscular,  or  emission  theory — Wave,  or  undulatory  theory,  356 — Sources  of 
light,  357— Similarity  of  light  and  heat,  358— Eelation  of  different  bodies  to  light- 
Luminous  and  non-luminous  bodies — Transparent,  translucent,  and  opaque  bodies, 
359 — A  medium — Propagation  of  light  in  a  homogeneous  medium,  360 — Absorption 
of  light,  361— Fig-.  1,  Rays,  pencils,  and  beams  of  light,  362— Fig-.  2,  Visible  bodies 
emit  light  from  every  point,  363 — Properties  of  light,  Absorption,  Dispersion,  Re- 
flection, and  Refraction,  364. 

CATOPTRICS,    OR   REFLECTION   OF  LIGHT. 

Reflectors — Mirrors— Specula — Reflectors,  365— Mirrors  and  specula,  366— Forms 
of  reflectors,  367 — Laws  of  reflection  of  light,  368 — Direction  in  which  objects  are 
seen,  369. 

Reflection  at  Plane  Surfaces — Fig-.  3,  Reflection  of  diverging  rays,  370— Fig-.  4, 
Reflection  of  converging  rays,  371 — Fig-.  5,  Reflection  of  parallel  rays,  372 — Fig-.  6, 
€onvex,  plane,  and  concave  mirrors,  373 — Fig-.  7,  Intensity  of  light  reflected  at 
different  angles,  and  from  different  surfaces,  374 — Fig-.  8,  Images  formed  by  plane 
reflectors — Virtual  image,  375 — Fig-.  9,  Multiplicity  of  images,  376 — Kaleidoscope,  377 
— Fig-.  10,  Deception  by  several  mirrors — Seeing  through  a  brick,  378 — Fig-.  11, 
Plane  mirrors  may  reflect  objects  double  their  own  length,  379 — Fig-.  12,  The  mari- 
ner's sextant,  380. 

Reflection  at  Curved  Surfaces — Fig-.  13,  Convex  spherical  mirrors  illustrated  by 
plane  mirrors,  381— Fig-.  14,  Convex  spherical  mirrors,  382— Apparent  size  of  ob- 
jects, 383— Fig-.  15,  Formation  of  images  by  convex  reflectors,  384— Images  formed 
by  convex  mirrors  are  smaller  the  nearer  the  object  approaches  the  mirror,  and 
vice  versa  (Fig.  15),  385— Fig-.  16,  Concave  reflectors  the  reverse  of  convex  reflec- 
tors, 386— Fig-.  17,  Formation  of  images  by  concave  reflectors,  387— Fig-.  18,  Foci 
of  concave  mirrors',  for  parallel  and  convergent  rays — Converging  rays  and  virtual 
focus  (Fig.  18),  388— Fig-.  19,  Foci  of  concave  mirrors  for  divergent  rays— Proper- 
ties of  conjugate  foci,  389— Secondary  axes— Oblique  pencils,  390— Fig-.  20,  Spheri- 
cal aberration  of  reflectors— Caustics,  391— Fig-.  21,  Parabolic  reflectors,  392— 
Fig-.  22,  Formation  of  images  by  concave  mirrors  when  the  object  is  beyond  the 
centre  of  curvature,  393. 

2 


18  CONTENTS. 

Dioptrics,  or  Refraction  of  Light— Fig.  23,  Definitions,  394— Laws  of  refraction, 
395— Causes  of  refraction,  396— Fig-.  24,  Eefraction  by  parallel  strata  of  different 
media,  397— Fig1.  25,  Eefraction  and  internal  reflection— Double  reflection  of  mir- 
rors, 398— Fig-.  26,  Refraction  and  total  reflection,  399— Fig-.  27,  Effects  of  refrac- 
tion' on  the  rising  and  setting  of  the  heavenly  bodies,  400— Fig-.  28,  Refraction  by 
dense  media  spreads  out  the  light,  401— Fig.  29,  Mirage,  402— Fig.  30,  Looming, 
403— Fig.  31,  The  depth  of  water  rendered  apparently  less  by  refraction,  404. 

Prisms  and  Lenses— Fig1.  32,  Different  kinds  of  prisms  and  lenses— Convergent 
and  divergent,  405 — Fig.  33,  Refraction  by  prisms— Finding  the  direction  of  the 
refracted  and  emergent  rays— Effects  of  a  plane-glass,  406— Fig-.  34,  The  course  of 
light  through  a  sphere  of  glass  or  spherical  lens,  407 — Fig.  35,  Action  of  convex 
lenses  on  light— Definitions,  408— Fig.  36,  Conjugate  foci,  409— Fig.  37,  Conjugate 
foci,  continued,  410 — Fig.  38,  Analogous  effects  of  prisms  and  double  convex  lenses, 
411 — Fig.  39,  Longitudinal  spherical  aberration  of  lenses — Determining  the  foci  of 
lenses— Plano-convex  lenses,  412— Fig.  40,  Formation  of  images  by  convex  lenses 
when  the  object  is  twice  the  focal  distance,  413— Fig.  41,  Formation  of  images, 
when  the  object  is  more  or  less  than  twice  the  focal  distance,  414— Fig.  42,  Formation 
of  images  when  the  object  is  at  less  than  the  focal  distance,  415 — Fig.  43,  Light- 
houses, 416 — Fig.  44,  Effects  of  concave  lenses  on  diverging,  parallel,  and  converg- 
ing rays,  417 — Fig.  45,  Formation  of  images  by  concave  lenses,  418. 

Chromatics  and  Decomposition  of  Light— Fig.  46,  Solar  spectrum— Primary 
colors,  419— Properties  of  the  solar  spectrum  (Fig.  46),  420— Complementary  colors, 
421— Analysis  of  colors  by  absorption,  422— Fig.  47,  Union  of  two  primary  colors 
to  produce  a  secondary  color,  423 — Fig.  48,  Composition  of  the  several  colors  of  the 
solar  spectrum,  424— Refraction  and  dispersion  of  the  solar  spectrum  (Fig.  48),  425 
—Dark  lines  in  the  solar  spectrum  (Fig.  48),  426— Lines  in  light  vary  with  different 
sources  of  light,  427— Fixed  lines  in  the  spectra  of  different  colored  flames,  428— 
Color  of  opaque  bodies,  429— Color  of  transparent  bodies,  430— Recomposition  of 
light,  431. 

The  Rainbow— Figs.  49  and  50,  Rainbows— primary  and  secondary,  432— Fig.  51, 
How  we  see  all  the  colors  of  the  rainbow  from  one  position,  433 — Fig.  52,  See  431 
— Fig.  53,  The  arch  of  the  rainbow — Width  of  the  bows  and  the  space  between 
them,  434— Fig.  54,  See  431. 

CHAPTER    XI. 

(CHART  NO.  6.) 
OPTICS,  CONTINUED,  AND  OPTICAL  INSTRUMENTS. 

Rainbows  further  Explained— Chromatic  Aberration— Fig-.  1,  Effects  of  a 
drop  of  water  upon  parallel  rays  of  light,  435 — Fog- bows,  Halos,  and  Coronas,  436 
— Fig.  2,  Chromatic  aberration,  437 — Fig.  3,  Achromatic  combination  of  lenses, 
438 — Fig.  4,  Recomposition  of  light  by  means  of  reversed  prisms  (see  431). 

Vision — Fig.  5,  The  camera  obscura,  439 — The  eye  a  camera  obscura,  440 — Fig.  6. 
Method  of  adjusting  the  pupil  of  the  eye,  441 — Fig1.  7,  Means  of  adjusting  and  hold- 
ing the  eye,  442 — Fig.  8,  Structure  of  the  interior  of  the  eye :  Sclerotic  coat — Cornea 
— Choroid  coat — Retina — Optic  nerve — Crystalline  lens — Aqueous  humor — Vitreous 
humor,  443 — Lachrymal,  or  tear  gland,  and  eye-lid,  444 — Fig.  9,  Adjustability  of  the 
eye  to  different  distances,  445 — Optical  axis,  446 — Optic  angle,  447 — Angle  of  vision 
(Fig.  8),  448 — Inversion  of  images  formed  in  the  eye,  449 — Why  we  see  objects  erect, 
their  images  being  inverted,  450 — The  brightness  of  the  ocular  image,  451 — Fig.  10, 
Indistinct  vision — Sufficiency  of  illumination,  452 — Fig.  11,  How  to  see  objects 
close  to  the  eye,  453 — Fig.  12,  Upon  what  brilliancy  of  vision  depends,  454 — Limit 
of  distinct  vision,  455 — Fig.  13,  Visual  rays  must  be  nearly  parallel,  456 — Size  of 
the  image  on  the  retina,  457 — Fig.  14,  Near-sightedness — Long-Sightedness,  458 — 
Fig.  15,  Near-sightedness  and  long-sightedness  caused  by  defective  form  of  the 
eyeball,  459 — Long-sightedness  of  old  persons,  460 — Conditions  of  distinct  vision, 


CONTENTS.  19 

46] — Sensibility  of  the  retina,  462 — Color-blindness,  463 — Effect  of  different  colors 
on  vision,  464 — Effects  of  background — Irradiation,  465 — Estimation  of  distance  and 
magnitude  of  objects,  466 — Why  with  two  eyes  we  see  objects  single,  467 — Double 
vision,  468 — Binocular  vision,  469 — Duration  of  impression  upon  the  retina,  470 — 
Optic  toys,  471 — Time  required  to  produce  visual  impressions,  472 — Sensations  of 
light  excited  by  other  causes  than  light,  473. 

Optical  Instruments — Variety,  and  principal  uses  of  optical  instruments,  474 — 
Spectacles,  475. 

Microscopes — The  simple  microscope  —Magnifying  power  of  lenses,  476 — Fig:.  16, 
Compound  microscope,  477— Fig.  17,  Magic  lantern,  478— Fig.  18,  Solar  microscope, 
479— Polyrama,  and  dissolving  views,  480— Fig-.  19,  Opera-glass,  481— Night-glasses, 
482. 

Camera  Obscura — Fig-.  20,  The  camera  obscura,  as  employed  for  tracing  land- 
scapes, etc.,  483 — Fig-.  21,  Another  form  of  the  camera  obscura,  484 — The  camera 
lucida,  485— Daguerreotyping,  486— Photography,  487. 

Telescopes— The  different  kinds  of  telescopes,  488— Fig-.  22,  The  refracting  astro- 
nomical telescope,  489— Fig-.  23,  The  terrestrial  telescope,  490— Fig.  24,  Herschel's 
reflecting  telescope,  491— Fig-.  25,  The  Gregorian  telescope,  492— Fig-.  26,  The  im- 
proved Newtonian  reflecting  telescope,  493 — Lord  Rosse's  reflecting  telescope,  494 — 
Fig-.  27,  The  telestereoscope,  495— Fig-.  28,  The  stereoscope,  496— Fig-.  29,  The  prin- 
ciples of  the  stereoscope,  497— Figs.  30  and  31,  The  stereomonoscope,  498. 

WAVE   THEORY   OF   LIGHT. 

Interference,  Diffraction,  etc.— Fig.  32,  Waves  of  light,  499— Fig.  33,  Direction 
of  vibrations  and  waves  of  light,  500 — Brilliancy  dependent  on  amplitude  of  waves. 
501 — Color  dependent  on  length  of  waves,  502 — Fig.  34,  Interference  of  light,  503 — 
Fig.  35,  Non-interference  of  light,  504— Fig.  36,  Demonstration  of  interference  of 
light,  505 — Laws  of  interference  and  non-interference  of  light,  506 — Fig.  37,  Inter- 
ference colors,  507— Figs.  38  and  39,  Determining  the  length  of  waves  of  light,  508 
— Length  of  waves  or  undulations  of  light — Table  for  the  different  colors,  509 — The 
cause  of  the  waves  of  light,  510 — Fig.  40,  Diffraction  fringes  caused  by  inter- 
ference, 511. 

Polarization  of  Light— Poles  in  physics,  512— Fig.  41,  Transmission  of  luminous 
waves,  513— Fig.  42,  Action  of  tourmaline  on  ordinary  light,  514 — Fig.  43,  Polariscope 
— Polarization  by  reflection,  515 — Plane  polarization,  516 — Waves  in  any  number  of 
planes  resolved  to  two  planes,  517 — Partial  polarization,  518 — Double  refraction,  519 
— Polarization  by  double  refraction,  520 — Useful  application  of  polarized  light,  521. 

Shadows — Fig-.  44,  Shadows  of  bodies  larger  than  the  illuminating  body,  522 — 
Fig.  45,  Shadows  of  bodies  smaller  than  the  illuminating  body,  523 — Umbra  and 
penumbra  (Fig.  45),  524— Fig-.  46,  Density  of  shadows,  525. 

Velocity  and  Intensity  of  Light— Fig.  47,  Velocity  of  light,  526— Fig.  48,  In- 
tensity of  light,  527— Fig.  49,  Photometers  :  Ritchie's— Eumford's— Silliman's— 
Bunsen's,  528— Fig.  50,  Intensity  of  light  at  different  distances,  529. 

CHAPTER    XII. 

(CHART  NO.  7.) 

ACOUSTICS. 

Production  and  Propagation  of  Sound — Definition,  530 — Sonorous  or  sounding 
bodies,  531 — Mediums,  532 — Sound  a  sensation,  533 — Different  sounds,  534 — Sonorous 
difference  .of  bodies,  535 — Time  is  required  for  the  transmission  of  sound,  536 — Cal- 
culation of  distances  by  sound,  537 — Velocity  of  all  sounds  the  same,  538 — Velocity 
of  sound  in  air,  539 — Velocity  of  sound  in  different  gases  and  vapors,  540 — Velocity 
of  sound  in  liquids,  541 — Velocity  of  sound  in  solids,  542 — Time  required  to  dis- 
tinguish sounds,  543. 


-20  CONTENTS. 

Reflection  of  Sound — Fig-.  1,  Reflection  of  sound  at  right  angles,  544 — Fig.  2, 
Sound  reflected  at  oblique  angles,  545 — Circular  waves  reflected  from  a  plane,  546 — 
Echoes,  547 — Fig.  3,  Multiple  echoes — Echoes  modify  the  tones  of  sound,  548 — Res- 
onance, 549 — Fig.  4,  Sound  reflected  in  a  sphere,  550 — Fig.  5,  Sound  propagated 
from  foci  of  ellipses,  551— Whispering  galleries,  552 — Audience  rooms,  553 — Fig.  6, 
Reflection  of  waves  by  parabolic  curves,  554. 

Intensity  of  Sound — Intensity  of  sound,  555 — Causes  which  modify  the  intensity 
of  sound,  556 — Intensity  of  sound  in  tubes,  557 — Fig.  7,  The  ear-trumpet,  558 — 
Fig.  8,  Speaking-trumpet,  559 — Fig.  9,  Vibrations  of  sonorous  bodies  illustrate^,  by 
the  Jews-harp,  560 — Fig.  10,  Sound  waves  caused  by  striking  a  bell,  561 — Fig.  11, 
Cause  of  vibrations  in  sonorous  bodies  illustrated  by  a  bell,  562 — Fig.  12,  Harmo- 
nicon,  563. 

Interference  of  Sound— Fig.  13,  Interference  of  sound,  564 — Combination  of  waves 
in  liquids,  565 — Fig.  14,  Interference  in  an  ellipse,  566 — Waves  of  condensation  and 
rarefaction,  567 — Interference  of  sound  waves — Co-existence  of  sonorous  waves,  568 
—  Undulation  of  solids,  569. 

Vibration  of  Cords — Fig.  15,  Elasticity  of  cords  and  wires  developed  by  tension, 
570— Fig.  16,  Nodal  points  of  vibrating  cords— Figs.  17  and  18,  Two  or  more  nodal 
points  in  one  string,  571 — Laws  of  the  vibration  of  cords,  572 — Fig.  19,  Verification 
of  the  laws  of  vibration  —The  Sonometer — Fig.  20,  Interference  of  sound  illustrated 
by  two  vibrating  cords — Fig.  21,  Interference  of  sound  further  illustrated,  573 — 
Fig.  22,  Sounds  caused  by  the  burning  of  hydrogen,  574 — Fig.  23,  Sound  not 
propagated  in  a  vacuum,  575. 

Vibration  of  Rods  and  Plates— Vibration  of  rods,  576— Means  of  vibrating  plates, 
577 — Nodal  lines  of  plates,  578 — Determination  of  nodal  lines  of  plates,  579 — 
Figs.  24,  25,  26,  27,  28,  29,  Nodal  points,  figures,  and  lines,  580— Fig.  30,  Refrac- 
tion of  sound,  581 — Laws  of  refraction  of  sound,  582. 

Sound  from  Pipes — Sound  from  pipes,  583 — Fig.  31,  Pipes  with  fixed  mouth-pieces, 
584— Reed  pipes,  585— Fig.  32,  Arrangement  of  reeds,  586— The  organs  of  voice  a 
reed  instrument,  587. 

Musical  Sounds — Difference  between  musical  sounds  and  noises,  588 — Qualities  of 
sound,  589— Limits  of  perceptible  sounds,  590— Unison,  591— Melody— Chord— Har- 
mony, 592— The  principal  harmonies,  593— The  most  pleasing  harmonies,  594— The 
limit  of  harmonies,  595— The  musical,  or  diatonic  scale— Gamut,  596— Formation  of 
the  musical  scale— Absolute  number  of  vibrations  corresponding  to  each  note,  597. 

CHAPTER    XIII. 

(CHART  NO.  7.) 

MAGNETISM. 

General  Properties  of  Magnets — Definition,  598 — Lodestone,  or  natural  magnets, 
599 — Fig.  33,  Magnetic  manifestations  of  lodestone,  600— Fig.  34,  The  armature, 
501 — Fig.  35,  A  fully-mounted  lodestone  magnet,  602 — Artificial  magnets,  603 — 
Fig.  36,  Method  of  making  an  artificial  magnet  with  lodestone,  604 — Fig.  37,  Dis- 
tribution of  force  in  magnets,  605 — The  law  of  distribution  of  attraction,  606 — The 
force  of  magnetic  attraction  at  different  distances,  607 — Effect  of  heat  on  magnets, 
608— Fig.  38,  Various  forms  of  magnets,  609— Fig.  39,  Compound  horse-shoe 
magnet,  610. 

Charging  Magnets— Methods  of  charging  magnets,  611— Fig.  40,  Method  of  charg- 
ing horse-shoe  magnets,  612 — Fig.  41,  Method  of  magnetizing  straight  bars,  613 — 
Fig.  42,  Both  poles  must  co-exist  in  every  magnet,  614 — Magnetic  and  magnetized 
bodies,  615. 

Magnetic  Induction — Fig.  43,  Induction — Magnetism  by  contact,  616 — Fig.  44, 
Magnetic  Induction  illustrated  by  a  series  of  rings,  617— Fig.  45,  Arrangement  of 
poles  in  a  star-shaped  body,  618 — Fig.  46,  Production  of  two  sets  of  poles  in  one  bar 


CONTENTS.  31 

by  induction,  619 — Fig1.  47,  Induction  without  contact,  620 — Fig.  48,  Magnets  do 
not  part  with  their  own  power,  621 — Fig-.  49,  Unlike  poles  neutralize  each  other, 
622— Fig.  50,  Neutralization  shown  by  the  Y-magnet,  623— Fig.  51,  The  inductive 
power  of  the  earth's  magnetism,  624. 

Hypothesis  and  Laws  of  Magnetism— Fig.  52,  Hypothesis  of  two  magnetic 
fluids,  625 — Laws  of  attraction  and  repulsion,  626 — The  coercitive  force,  627 — 
Fig-.  53,  Magnetic  curves  rendered  apparent  to  the  eye,  628 — Fig.  54,  Curves  with 
two  magnets  and  unlike  poles,  629 —  Fig.  55,  Curves  with  two  magnets  and  similar 
poles,  630 — Magnetic  attraction  not  intercepted,  631 — Preservation  of  magnets,  632. 

Terrestrial  Magnetism— The  earth  as  a  magnet,  633— Fig.  56,  The  astatic  needle, 
634— Fig.  57,  Magnetic  needle,  635— Directive  force  of  magnets— The  directive  force 
simply  rotates  the  magnet,  or  needle,  636 — Magnetic  meridian,  637 — Variations  of 
the  needle— Declination,  638— Daily,  annual,  and  other  variations  of  the  needle,  639— 
Inclination  or  dip  of  the  needle,  640 — Fig.  58,  Action  of  the  earth  illustrated  by  the 
action  of  a  magnet,  641— Fig.  59,  Dipping  needle,  642— Fig.  60,  Position  of  the 
dipping  needle  in  different  parts  of  the  earth,  643 — Fig.  61,  The  mariner's  compass, 
644 — Table  for  correcting  the  variations  of  the  compass — Discovery  of  the  compass 
— Magnetic  intensity,  645 — The  inductive  power  of  the  earth's  magnetism,  646 — 
Utilization  of  magnetism,  647. 

CHAPTER    XIV. 

(CHART  NO.  8.) 

ELECTRICITY. 

STATICAL  OR  FRICTIONAL  ELECTRICITY. 

Fundamental  Principles— Definitions,  648— Discovery  of  electricity,  649— The 
sources  of  electricity,  650 — Fig.  1,  Electrical  effects,  651 — Electroscope — Electrical 
pendulum,  652 — Fig.  2,  Vitreous  and  resinous,  or  positive  and  negative  electricities, 
653— The  theory  of  two  fluids,  654— The  single  fluid  hypothesis— The  term  fluid,  655 
— Fig.  3,  Attraction  and  repulsion,  656 — Laws  of  electrical  attraction  and  repulsion, 
657 — Conductors  of  electricity,  658 — Insulators,  659 — The  earth  is  the  great  reservoir, 
660 — Method  of  electrifying  bodies,  661 — Electrical  tension,  662 — Fig.  4,  Electricity 
accumulates  only  on  the  outer  surfaces  of  bodies — Fig.  5,  The  same  fact  shown  in  a 
different  way,  663 — Proof-plane,  664 — Fig-.  6,  Distribution  dependent  on  form,  665 — 
The  power  of  points,  666 —The  loss  of  electricity  in  excited  bodies,  667. 

Induction  of  Electricity— Fig.  7,  Bodies  electrified  by  induction,  668— Fig-.  8,  The 
two  fluids  separated  and  obtained  by  induction — Laws  of  electrical  induction,  669 — 
Fig.  9,  Dielectrics — Explanation  of  induction,  670 — Attraction  and  repulsion  of 
light  bodies  explained,  671 — ELECTROMETERS:  Electroscope,  672 — Fig.  10,  The  quad- 
rant electrometer,  673 — Fig.  11,  The  gold-leaf  electrometer,  674 — Method  of  using 
the  gold-leaf  electrometer,  675. 

Electrical  Machines— Figs.  12  and  13,  The  electrophorus,  676— Fig.  14,  The  cyl- 
inder electrical  machine,  677 — Fig.  15,  The  plate  electrical  machine,  678 — Use  of 
the  electrical  machines,  679— Measure  of  the  quantity  of  electricity  in  the  machine, 
680— Precautions  in  using  the  machines,  681— Fig.  16,  The  hydro-electric  machine, 
682 — Other  sources  of  electrical  excitement,  683. 

Experiments  Illustrating  Electrical  Attraction  and  Repulsion— The  insulat- 
ing stool— Electrical  spark— Electrical  shock,  684— Fig.  17,  Electrical  puppets,  680 
—Fig.  18,  The  electrical  chime,  686— Fig.  19,  The  electrical  wheel,  687— Fig.  20, 
The  electrical  blow-pipe,  688— Fig.  21,  The  electrical  egg,  produced  by  passing 
electricity  through  a  vacuum,  689. 

Accumulation  of  Electricity — Latent  or  disguised  electricity,  690 — The  electrical 
condenser,  691— Fig.  22,  The  Leyden  jar,  692— Fig.  23,  Charging  the  Leyden  jar, 
693—  Limit  of  the  charge  in  a  condenser,  694— DISCHARGING  THE  JAR;  Disruptive  dis- 
charge, 694 — Insensible  or  gradual  discharge — Discharge  by  small  and  sudden  dis- 


22  CONTENTS. 

charges — Instantaneous  discharge,  695 — DISCHARGERS  (Fig.  22) :  The  discharging 
rod,  or  hand  discharger,  696 — Fig.  24,  The  universal  discharger,  697 — Electricity  in 
the  Leyden  jar  resides  on  the  glass,  698— Fig".  25,  The  electric  battery,  699— Fig.  26, 
The  electric  spark,  700— The  color  of  the  electric  spark,  701— Fig.  27,  Difference 
between  the  positive  and  negative  spark,  702 — Fig.  28,  The  electrical  square,  703. 

Effects  of  Accumulated  Electricity — EFFECTS  OF  THE  ELECTRIC  DISCHARGE,  704 — 
Physiological  effects,  705 — Heating  power  of  electricity,  706 — The  mechanical  effects 
of  electricity,  707 — The  chemical  effects  of  statical  electricity,  708. 

Atmospheric  Electricity — Franklin's  experiment  with  a  kite,  709 — Free  electricity 
in  the  atmosphere,  710 — Causes  of  atmospheric  electricity,  711 — Thunder  storms — 
Origin  of  thunder  clouds,  712 — Thunder,  713 — Lightning,  714 — CLASSES  OF  LIGHTNING: 
Zigzag,  or  chain  lightning — Sheet  lightning — Heat  lightning — Volcanic  lightning, 
715— Velocity  of  lightning,  716— The  return  shock,  717— Fig.  29,  Lightning-rods- 
Other  means  of  safety — Liability  of  being  struck  by  lightning,  718 — Aurora  borealis, 
719_Fig.  30,  Slow  discharge  of  a  Leyden  jar— A  beautiful  toy,  720. 

CHAPTER    XV. 

DYNAMICAL  ELECTRICITY. 
FUNDAMENTAL  PRINCIPLES. 

Fundamental  Principles— Galvanism,  721— Fig.  31,  Galvani's  discovery  and  ex- 
periments, 722 — Galvani's  explanation,  723 — Volta's  theory  of  contact — Volta's  dis- 
covery, 724 — The  electro-chemical  theory,  725 — Fig.  32,  Simple  Voltaic  couple,  726 
—Fig.  33,  The  Voltaic  pile  or  battery,  727— Varieties  of  Voltaic  piles,  728— Polarity 
of  the  pile,  729 — Electrical  currents  of  the  pile,  730 — Electro-positive  and  electro- 
negative, 731 — The  difference  between  quantity  and  intensity,  732— Quantity  in- 
creases with  surface,  intensity  with  number  of  pairs,  733 — Amalgamated  zinc,  734. 

Batteries— Smee's  battery,  735— Fig.  34,  Sulphate  of  copper  battery,  736— Fig.  35, 
Bohnenberger's  electroscope,  737 — Fig.  36,  Grove's  nitric  acid  battery,  738 — Fig.  37, 
Carbon  battery,  739— Fig.  38,  Batteries  of  two  or  more  couples,  740— The  electro- 
motive force,  741.  — Eesistance  to  the  current,  742— Laws  determining  the  force  of  a 
Voltaic  current,  743 — Difference  between  static  and  dynamic  electricity,  744. 

Applications  of  Voltaic  or  Galvanic  Electricity— THE  EFFECTS  OF  THE  VOLTAIC 
BATTERY,  745— Physical  effects:  Fig.  39,  Illuminating  effects,  746— Fig.  4O,  The 
Voltaic  arch,  747— Fig.  41,  The  oval  form  of  the  arch,  748— Fig.  42,  The  shape  of 
the  electrodes,  749— -Properties  of  the  electric  light,  750— Fig.  43,  Heating  effects- 
Deflagration,  751 — Chemical  effects:  Decomposition,  752 — Fig.  44,  Method  of  electro- 
typing — Preparing  the  mould — Method  of  depositing  the  metal  on  the  mould,  753 — 
Electro-gilding  and  electro-plating,  754 — Fig.  45,  Voltaic  decomposition  of  water, 
755 — Fig.  46,  Decomposition  of  salts,  756 — Quantity  of  electricity  required  to  pro- 
duce chemical  action  is  enormous,  757 — Physiological  effects  of  the  Voltaic  current, 
758. 

CHAPTER    XVI. 

ELECTRO-DYNAMICS. 

Electro-magnetism— Relation  between  magnetism  and  electricity,  759— Ersted's 
discovery,  760— Fig.  47,  Action  of  an  electric  current  upon  a  magnet  or  needle,  761 
— Fig.  48,  Galvanometers,  or  multipliers,  762 — The  directive  action  of  the  earth,  763 
—Fig.  49,  The  astatic  needle,  764— The  electro-magnetic  force  is  lateral  and  tan- 
gential to  the  electric  current,  765 — Ampere's  electro-magnetic  theory,  766 — Fig.  5O. 
Mutual  action  of  electric  currents,  767— Fig.  51,  Attraction  of  currents,  768— Fig.  52, 
Action  of  magnets  upon  currents,  769— Fig.  53,  A  single  helix,  770— Fig.  54,  A 
double  helix,  771— Fig.  55,  Magnetizing  by  the  helix  and  electrical  current,  772— 
Fig.  56,  Electro-magnets,  773 — Bodies  suspended  without  contact,  774 — Utilization 
of  electro-magnetic  force,  775. 

The  Electric  Telegraph— First  experiments  in  electrical  telegraphing,  776 — Fig.  57, 


CONTENTS.  23 

Morse's  recording  telegraph — The  receiving  or  recording  instrument — The  alphabet 
— The  instrument  for  transmitting  the  message — House's  telegraph,  or  printing  tele- 
graph, 777 — The  earth's  circuit — Insulators,  778. 

Electro-dynamic  Induction — Magneto-electricity — Thermo-electricity,  etc. 
— Fig.  58,  The  revolving  electro-magnet,  779 — Fig.  59,  Cause  of  the  earth's  magnet- 
ism, 780— Fig-.  60,  Magneto-electricity,  781— Magneto-electric  Machines,  782— Fig- 
ures 61  and  62,  Diamagnetism,  783 — INDUCTION  BY  CURRENTS  :  Fig.  63,  Currents  in- 
duced by  other  currents,  784— Fig.  64,  Induced  currents  of  different  orders,  785— The 
properties  of  induced  currents,  786— Fig.  65,  Thermo-electricity,  787— Fig.  66,  The 
thermo-electric  revolving  arch,  788. 

Organic  Electricity— Animal  electricity,  789— Electrical  animals,  790— Electricity 
of  plants,  791. 

CHAPTER    XVII. 

(CHART   NO.  9.) 
.       ASTRONOMY. 

Definitions,  Introductory  Observations,  and  Theories— Astronomy,  792— GEN- 
KRAL  DIVISIONS  OF  THE  SUBJECT  :  Descriptive  astronomy — Physical  astronomy — Prac- 
tical astronomy,  793 — Different  classes  of  heavenly  bodies,  794 — Extent  of  space,  795 
— Magnitude  of  heavenly  bodies,  796 — The  number  of  heavenly  bodies,  797 — Dis- 
tances between  heavenly  bodies,  798 — The  orbital  motions  of  heavenly  bodies,  799 
— The  velocity  of  heavenly  bodies,  800 — Early  observations  of  astronomical  phenom- 
ena, 801  —  Ptolemy's  great  system,  802—  Copernicus'  theory,  803— Kepler's  dis- 
coveries and  laws,  804 — Galileo's  discoveries,  805 — Newton's  discovery,  806. 

The  Solar  System— CLASSIFICATION— Fig.  1,  The  solar  system— Planets,  primaries 
and  secondaries — Interior  and  exterior  planets — Comets — Solar  bodies,  807 — Fig.  2, 
Relative  magnitude  of  the  planets,  808 — Fig.  3,  Approximate  relative  distances  of  the 
planets,  809 — Impossibility  of  delineating  the  solar  system — Solar  system  repre- 
sented by  real  objects — Representation  of  the  motion  of  the  planets,  810. 

The  Sun— Influence  of  the  sun,  811— Magnitude  of  the  sun,  812— The  distance  of  the 
sun  from  the  earth,  813 — Telescopic  view  of  the  sun — Dark  spots — Motions  of  the 
sun,  814. 

The  Primary  Planets — Periodic  revolutions,  815 — Velocity  of  the  planets,  816 — 
Diurnal  revolution  of  the  planets,  817 — Magnitude  of  the  planets,  818 — Relative 
magnitude  of  the  planets,  819 — The  distances  of  the  planets  from  the  sun,  820 — 
Density  of  the  planets,  821— Attraction  of  the  planets,  822— Light  and  heat  of  the 
planets,  823— The  ASTEROIDS— Table  of  the  asteroids,  824. 

The  Secondary  Planets  or  Satellites — Compound  motion  of  the  satellites,  825 — 
The  earth's  satellite  or  moon,  826 — Jupiter's  satellites  (Figs.  1  and  3) — Their  diam- 
eters, distances,  and  periodic  times,  827 — Saturn's  satellites  (Figs.  1,3,  and  9) — Their 
distances  and  periodic  times,  828 — Uranus'  satellites  (Figs.  I  and  3) — Their  distances 
and  periodic  times,  829 — Neptune's  satellite  (Figs.  1  and  3) — Its  distance  and  peri- 
odic time,  830. 

Comets — Nature  of  comets,  831 — Orbits  and  velocity  of  comets  (Fig.  1),  832 — Periodic 
times  of  comets,   833 — The  number  of  comets,  834 — The  direction  of  motions  of 
comets,  835. 
A  few  particulars  relating  to  the  telescopic  views  of  the  primary  planets,  836. 

Orbits,  Eccentricity  of  Orbits,  etc. -Fig.  4,  Orbits  are  elliptical,  837— The  ec- 
centricity of  a  planet's  orbit — The  eccentricity  of  the  different  orbits,  838 — Aphelion 
and  Perihelion,  839 — The  radius  vector  (Fig.  4),  840 — The  radius  vector  passes  over 
equal  areas  in  equal  times,  841 — Fig.  5,  Circular  motion,  842 — Centripetal  and  cen- 
trifugal forces,  843 — Why  the  planets  do  not  fall  to  the  sun,  844 — Centre  of  gravity 
and  motion  of  the  solar  system,  845— Planes  of  orbits,  846— Fig.  6,  The  ecliptic,  847 
— Obliquity  of  the  ecliptic,  848 — Inclination  of  orbits  of  planets  to  the  plane  of  the 
ecliptic  (Fig.  6),  849— Fig.  7,  The  figure  or  form  of  the  planets,  850— Fig.  8,  Venus 


24  CONTENTS. 

as  morning  and  evening  star,  851— Figures  9  and  10,  Saturn's  rings,  852— Fig.  11, 
Distances  of  the  satellites  from  their  primaries,  853  —Fig.  12,  Solar  and  sidereal 
time,  854. 

The  Moon Its  Path,  Phases,  etc. — Fig.  13,  The  moon's  path   around  the  sun, 

855 — Sidereal  and  synodic  revolution  of  the  moon,  856 — The  rotation  of  the  moon  on 
her  axis,  857 — The  moon's  librations  in  latitude  and  longitude,  858 — Fig.  14,  The 
actual  path  of  the  moon — The  motion  of  the  moon  is  never  retrograde,  859 — Fig.  15, 
The  moon's  orbit  always  concave  toward  the  sun,  860 — View  of  the  earth  from  the 
moon,  861 — Fig.  16,  The  moon's  phases — Importance  of  the  phases  and  motions  of 
the  moon,  862 — Why  the  dark  side  of  the  moon  is  visible  near  conjunction,  863 — 
Other  particulars  relating  to  the  moon,  864. 

CHAPTER    XVIII. 

(CHART  NO.  10.) 

ASTRONOMY. 

ZODIAC,   SEASONS,   ECLIPSES,    TIDES,   FIXED  STARS,   ETC. 

The  Zodiac  and  Philosophy  of  Seasons — Fig.  1,  The  zodiac,  865— The  signs  or 
constellations  of  the  zodiac,  866 — Day  and  night  (Fig.  1),  867 — Causes  of  the  seasons, 
(Fig.  1),  868— The  earth  at  the  solstitial  points  (Fig.  1),  869— The  earth  at  the  equi- 
noctial points  (Fig.  1),  870— The  sun's  declination,  871  and  931— Constellations  of 
the  zodiac  (Fig.  1),  872 — The  sun's  apparent  motion  in  the  ecliptic  (Fig.  1),  873 — 
Division  of  the  signs,  874 — The  recession  of  the  equinoxes  or  precession  of  the  con- 
stellations, 875 — Longitude  in  the  heavens,  876 — Fig.  2,  Intersection  of  the  ecliptic 
and  equinoctial,  877 — Polar  inclination  and  seasons  of  the  different  planets,  878. 

The  Philosophy  of  Transits,  etc.— Transits— Nodes,  879— Fig.  3,  Transits  of  Mer- 
cury, 880 — The  calculation  of  transits  and  eclipses,  881 — Fig.  4,  Mercury's  oscilla- 
tion, 882— Fig.  5,  Inclination  of  the  moon's  orbit  to  the  plane  of  the  ecliptic,  883— 
View  of  the  moon  at  the  poles  and  at  the  equator,  884. 

Parallax  of  the  Heavenly  Bodies,  Conjunction,  etc.— Fig.  6,  Annual  parallax, 
or  parallax  of  the  stars,  885— Fig.  7,  Diurnal  parallax,  886— The  effect  of  parallax  on 
bodies,  887— The  principles  of  parallax  of  great  importance,  888— Fig.  8,  Convexity 
of  the  Earth's  surface,  how  shown,  889 — Fig.  9,  Conjunction  and  opposition  of  plan- 
ets, 890 — Direct,  stationary,  and  retrograde  motion  of  planets  (Fig.  9),  891 — The  tran- 
sit of  Venus,  an  important  event,  892— Transits  of  Venus  from  1639  to  2012—  Fig.  10, 
The  periodic  revolution  of  the  sun,  893. 

CHAPTER  XIX. 

Philosophy  of  Eclipses— Shadows  of  solar  bodies,  894— Interest  felt  in  eclipses,  895 
— Position  of  sun,  earth,  and  moon  when  eclipses  occur,  896 — Eclipses  are  either 
total,  partial,  or  annular,  897 — Fig.  11,  The  direction  in  which  eclipses  come  on,  898 
— Total  eclipse  of  the  moon,  and  partial  eclipse  of  the  sun  (Fig.  11),  899 — Dimen- 
sions of  the  earth  and  moon's  shadows,  900 — Fig.  12,  Total  and  annular  eclipses  of 
the  sun,  901 — Duration  of  eclipses,  902 — The  general  effects  of  a  total  eclipse  of  the 
sun,  903 — The  number  of  eclipses  in  any  one  year,  904 — Fig.  13,  Why  eclipses  are 
not  more  frequent,  905 — Retrograde  motion  of  the  moon's  nodes,  906 — Fig.  14,  The 
solar  and  lunar  ecliptic  limits,  907 — Why  there  are  more  solar  than  lunar  eclipses, 
908 — Eclipses  or  occultation  of  the  stars,  909 — Eclipses  of  Jupiter's  moons,  910 — 
Eclipses  of  Saturn's  moons,  911. 

CHAPTER  XX. 

Philosophy  of  the  Tides— Motion  of  the  water  of  the  earth,  912— The  tides  are  not 
uniform,  913 — The  principal  cause  of  the  tides,  914 — Fig.  15,  Influence  of  the  earth 
upon  its  waters,  915— Fig.  16,  A  single  tide-wave,  916— Fig.  17,  The  two  tide- 


CONTENTS.  25 

waves,  917 — Fig.  18,  Lagging  of  the  tide-wave  behind  the  moon,  918 — Fig.  19,  In- 
fluence of  the  sun  upon  tides,  919 — "Fig.  20,  Causes  of  the  opposite  tide-wave,  920 
— The  secondary  cause  of  the  opposite  tide-wave  (Fig.  20),  921 — Kelative  influence 
of  the  sun  and  moon  on  the  tides,  922 — Fig.  21,  Spring  and  neap  tides,  923 
— Variations  in  the  spring  tides  (Fig.  21),  924 — Tides  affected  by  declination,  925 — 
OTHER  CAUSES  AFFECTING  TIDES:  The  winds  affect  the  tides,  926— -The  conformation 
of  the  land  affects  tides,  927— The  average  elevation  of  tides,  928— The  different 
heights  of  water  in  different  oceans  and  seas,  929 — Atmospherical  tides,  930. 

Sun's  Declination,  Zones,  and  Temperature— Fig.  22,  The  declination  of  the 
sun  differently  illustrated,  931 — The  zones— The  torrid  zone — The  frigid  zones — The 
temperate  zones,  932 — "When  the  sun  shines  on  the  poles  (Fig.  22),  933 — The  effect  of 
the  sun's  declination  on  temperature,  934. 

Terrestrial  and  Celestial  Globes— LATITUDE  AND  LONGITUDE  :  Figr.  23,  Celestial 
and  terrestrial  latitude,  935 — Celestial  and  terrestrial  longitude,  936 — The  terres- 
trial globe,  937— THE  CELESTIAL  GLOBE  (Fig.  23),— The  celestial  poles— The  plane  of  a 
meridian — The  right  ascension  of  a  body — The  angle  of  right  ascension — Circles  of 
celestial  latitude — The  angles  of  longitude — The  celestial  horizon — The  sensible  ho- 
rizon— Vertical  circles — The  meridian — The  prime  vertical  circle — Zenith  distance 
—The  azimuth— Amplitude,  938— Nutation  of  the  earth's  ax'is  (Fig.  23),  939. 

The  Fixed  Stars — Motion  of  the  stars,  940 — Variable  or  periodical  stars,  941 — Tem- 
porary or  "new  and  lost  "  stars,  942 — Double  stars,  943 — Binary  systems,  944 — Clus- 
ters of  stars,  945— Nebulae,  946—  Classes  of  Nebulce  :  1,  Resolved;  2,  Resolvable;  3, 
Stellar;  4,  Irresolvable;  5,  Planetary,  947 — The  milky  way  an  annular  nebula, 
948 — The  number  of  stars,  949 — The  term  Universe,  950 — Our  Cluster  or  Firmament, 
951. 


INTRODUCTION. 


CLASSIFICATION  OF  THE  SCIENCES. 

A  law  is  a  necessary  relation  between  cause  and  effect ;  universal 
experience  having  shown  that  like  causes  always  produce  like  effects. 

General  Science  is  a  knowledge  of  the  laws  of  the  Universe. 

A  special  science  consists  of  the  collection,  classification,  and  explana- 
tion of  all  the  known  laws  and  leading  truths  relating  to  some  definite 
subject.  For  example :  the  Science  of  Astronomy  is  made  up  of  the 
collection,  classification,  and  explanation  of  all  the  known  laws  and 
leading  truths  which  relate  to  the  heavenly  or  celestial  bodies. 

Knowledge  which  relates  to  Mind  is  called  Science  of  Mind,  or  META- 
PHYSICS; and  is  subdivided  into  Intellectual  Science,  Moral  Science, 
Science  of  Logic,  etc. 

Knowledge  which  relates  to  the  Material  Universe  is  called  Physical 
Science,  or  Natural  Philosophy  ;  which  is  subdivided  into  Science  of 
Organized  Matter,  or  PHYSIOLOGY,  and  Science  of  Unorganized  Matter, 
or  GENERAL  PHYSICS. 

Physiology  treats  of  matter  as  modified  by  the  force  or  principle  of 
vitality,  and  is  further  divided  into  two  branches:  Animal  Physiology, 
or  ZOOLOGY,  and  Vegetable  Physiology,  or  BOTANY. 

Unorganized  matter  is  divided  into  two  classes,  Celestial  and  Terres- 
trial. Hence  General  Physics  treats  of  celestial  bodies  (including  the 
earth  as  a  whole),  called  Astronomy,  and  terrestrial  bodies,  called  Ter- 
restrial Physics. 

Terrestrial  Physics  is  again  subdivided  into  two  branches,  called 
Physics  (or  Natural  Philosophy)  and  Chemistry.  The  former  treats  of 
the  general  properties  of  bodies  ;  the  latter  treats  of  the  ultimate  par- 
ticles of  bodies  and  their  laws  of  combination. 

For  a  further  explanation  of  the  relation  between  the  science  of 
Chemistry  and  that  of  Physics  (or  Natural  Philosophy),  see  paragraphs 
2,  6,  7,  and  8. 


HANDBOOK 


OF 


NATURAL   PHILOSOPHY. 


CHAPTER    I. 

(CHART  NO.  1.) 

MATTER,   FORCE,    MOTION",   AND   MECHANICS. 
Definitions  and  Preliminary  Principles. 

1,  Matter. — Matter  is  the  general  name  of  everything  that  occupies 
space,  and  which,  in  an  infinite  variety  of  forms,  is  the  object  of  sense. 
It  is  only  through  the  agency  of  our  five  senses  that  we  become  con- 
scious of  the  existence  of  any  matter,  even  of  our  own  bodies. 

A  body  is  a  definite  and  limited  portion  of  matter,  be  it  a  world,  or  a 
particle  of  dust. 

Different  kinds  of  matter,  as  iron,  granite,  or  water,  are  called  sub- 
stances. Though  there  are  a  vast  number  of  different  substances,  there 
have  been  found,  by  chemical  analysis,  as  yet,  only  about  sixty-four 
different  kinds  of  matter,  termed  elements, — some  ten  or  twelve, 
only,  of  these  making  up  the  great  bulk  of  all  we  see. 

Some  bodies  or  substances  consist  of  a  single  element,  as  oxygen, 
carbon,  iron,  sulphur,  gold,  etc. ;  others  of  two  or  more  elements,  as 
water,  consisting  of  two  (hydrogen  and  oxygen),  and  oil,  three ;  crystal- 
lized common  salt,  four ;  crystallized  alum,  five ;  pure  white  of  eggs,  six. 

2.  Changes  in  matter,  chemical  or  physical.— The  peculiar 
attraction  which    draws  and  unites  together  the  ultimate  atoms  of 
different  simple   elements   is  termed   chemical  affinity.     The   simple 
elements,  when  united  by  this  affinity,  become  entirely  changed  in 
their  physical  properties ;  for  instance,  oxygen,  which  is  a  gas,  and  the 
best   supporter  of  combustion,  and  hydrogen,  also  a  gas,  and  the  most 
inflammable  and  lightest  element,  when  chemically  combined,  instead 


30  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

of  remaining  gases  (or  forming  a  new  gas)  and  affording  a  substance 
for  rapid  combustion  with  intense  heat,  as  might  be  expected,  are  so 
modified  by  the  chemical  affinity  which  unites  them,  that  water,  a 
dense  and  unelastic  liquid  which  extinguishes  fire,  is  the  result.  And 
though  the  specific  identity  of  these  two  elements  is  wholly  destroyed 
by  this  chemical  union,  yet  the  ultimate  atoms  are  not  changed.  Such 
changes,  destructive  of  specific  identity,  are  called  chemical  changes. 

Changes  which  do  not  destroy  specific  identity  are  termed  physical 
changes ;  as  when  an  iron  bar  acquires  magnetism  from  loadstone,  or 
when  a  glass  tube  becomes  electrical,  by  being  rubbed  with  silk ;  or,  as 
in  the  case  of  water,  which,  being  deprived  of  a  portion  of  its  heat, 
becomes  a  solid,  or,  by  an  increase  of  heat,  is  changed  to  steam  or 
vapor,  when  again  it  returns  to  the  earth,  as  dew,  mist,  rain,  hail,  or 
snow,  and  so  back  to  its  liquid  form.  But  water,  through  all  these 
changes  of  state  and  position,  is  still  the  same  substance,  having  lost 
none  of  its  properties. 

3.  Light,  heat,  and  electricity. — These  may  be  considered 
agents  or  forces  connected  with  or  growing  out  of  the  changes  of 
matter,  physical  or  chemical,  or  both.  Or,  as  most  generally  believed, 
they  may  depend  on  the  existence  of  certain  hypothetical  fluids,  or  on 
the  vibrations  of  an  assumed  ethereal  medium. 

As  these  fluids,  forces,  or  agents,  are  without  weight  and  other 
sensible  properties  of  grosser  or  denser  matter,  they  are  termed  im- 
ponderables. These,  as  it  were,  are  the  life  and  spirit  of  matter. 

Jf,.  Atoms. — There  is  a  difference  of  opinion  about  the  ultimate 
constitution  of  matter.  The  general  belief  is,  that  matter  is  formed  of 
ultimate  particles,  which  are  movable,  solid,  impenetrable,  and  so  hard 
as  never  to  wear  or  break  in  pieces;  having  a  certain  definite  size, 
figure,  and  weight,  which  they  retain  unchangeable  through  all  their 
various  combinations.  These  are  called  atoms, — signifying  that  which 
cannot  be  divided.  Their  sizes,  in  different  elements,  are  supposed  to 
vary,  though  the  largest  of  them  are  inconceivably  small ;  and  their 
forms  may  be  very  various,  though  considered  to  be  generally  globular. 

5.  Molecules. — The  term  molecule  (a  little  mass)  is  more  com- 
monly applied  to  what,  in  chemistry,  are  termed  divisible  atoms ;  that 
is,  to  a  group  of  two  or  more  atoms  of  different  elements;  as,  for 
instance,  a  molecule  of  water  is  composed  of  at  least  two  atoms,  one  of 
oxygen  and  one  of  hydrogen,  forming  a  chemical  compound. 

However  small  the  various  ultimate  atoms  are,  their  oval  form 
affords  space  around  about  and  between  them ;  while  molecules  are 


PRELIMINARY  PRINCIPLES.  31 

supposed  to  touch  each  other,  if  at  all,  only  at  a  few  points,  thus 
affording  interspaces  larger  than  their  own  bulk,  which  accounts  for 
the  two  general  properties  of  matter,  termed  compressibility  and 
expansibility. 

6.  The  properties  of  matter  are  general  or  specific.— 

Gold,  for  example,  occupies  space  and  possesses  weight,  so  also  does  all 
matter,  whether  solid,  liquid,  or  gaseous ;  hence  these  properties  are 
general.  But  its  color,  lustre,  crystalline  form,  and  other  peculiarities 
that  distinguish  it  from  other  substances,  are  specific  properties. 

7.  Physical   and  chemical  properties   of  matter. — The 

chemical  and  physical  changes  of  matter  (2)  above  described,  corre- 
spond to  its  chemical  and  physical  properties.  The  specific  properties 
of  gold  depend  solely  upon  its  physical  qualities.  Density,  lustre,  color, 
form,  malleability,  and  its  high  point  of  fusion,  are  all  qualities  of  gold 
which  can  never  be  lost  without  an  essential  change  of  its  nature,  and 
are,  therefore,  termed  physical  properties.  Exposed,  however,  to  the 
action  of  chlorine  and  certain  other  agents,  gold  loses  its  specific  iden- 
tity, and  becomes,  as  it  were,  a  new  substance,  while  the  same  change 
passes  equally  upon  the  agent  by  whose  efficiency  the  transmutation  is 
effected,  thus  destroying  the  essential  specific  identity  of  both  sub- 
stances. These  changes  are  the  result  of  chemical  affinity,  which  de- 
pends upon  the  chemical  properties  of  matter. 

8.  Physics,  or  Natural  Philosophy,  and  Chemistry.— The 

foregoing  fundamental  distinctions  between  the  physical  and  chemical 
changes,  and  between  the  physical  and  chemical  properties  of  matter, 
show  the  distinction  between  Natural  Philosophy  and  the  science  of 
Chemistry  ;  but  as  all  substances  possess  both  physical  and  chemical 
properties,  it  is  evident  that  a  thorough  acquaintance  with  either  of 
these  branches  of  knowledge  involves  some  familiarity  with  the  other. 
The  object,  then,  of  Natural  Philosophy,  or  Physics,  is  the  investi- 
gation of  the  general  properties  of  unorganized  bodies,  and  of  their 
action  on  each  other. 


32  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

CHAPTER    II. 

DEFINITIONS   AND    GENEEAL    PROPERTIES   OF   MATTER. 

The  essential  properties  of  matter. — These  are  magnitude, 
or  extension,  and  impenetr 'ability. 

0.  By  magnitude,  or  extension,  is  meant  the  property  which 
every  body  possesses  of  occupying  a  portion  of  space.  The  amount  of 
space  occupied  is  termed  its  volume.  Every  body,  however  small,  has 
three  dimensions — length,  breadth,  and  thickness. 

10.  By  impenetrability  is  meant  that  property  of  matter  which 
renders  it  impossible  for  two  separate  bodies  to  occupy  the  same  space 
at  the  same  time.     Some  bodies,  like  air,  may  be  compressed  almost 
indefinitely,  but  the  power  required  to  do  it,  becomes  the  evidence  and 
the  measure  of  its  impenetrability.    A  nail  driven  into  wood,  or  a  stone 
dropped  into  water,  or  a  ball  thrown  through  the  air,  are  instances  of 
displacement,  and  not  penetrability. 

Secondary  or  accessory  properties  of  matter.— These  are 

Divisibility,  Compressibility,  Expansibility,  Porosity,  Mobility,  Inertia, 
Indestructibility,  and  Attraction. 

11.  Divisibility.— By  divisibility  of  matter  is  meant  that  a  body 
may  be  divided  into  two  parts,  and  that  these  parts  may  again  be 
divided  into  other  parts,  and  so  on,  until  the  parts  become  infinitesi- 
mally  small.     Suppose  a  bit  of  marble  (carbonate  of  lime)  to  be  thus 
divided,  and  when  the  smallest  imaginable  particle  has  been  reached, 
it  can  be  still  further  divided  by  chemical  decomposition  into  three 
elements :  first  into  carbonic  acid  and  lime,  then  the  former  of  these 
into  carbon  and  oxygen,  and.  the  latter  into  calcium  and  oxygen. 

Some  idea  of  the  extreme  divisibility  of  matter  may  be  obtained  by 
the  fact  that  a  single  grain  of  musk  will  scent  a  large  hall  for  many 
years  and  lose  no  appreciable  part  of  its  weight.  Again,  many  kinds 
of  animalcules,  as  well  adapted  to  life  as  the  largest  beasts,  are  so 
small  that  hundreds  of  thousands  might  swim  side  by  side  through 
the  eye  of  a  small  needle ;  yet  each  of  these  is  a  fully  organized  being. 
How  minute,  then,  must  be  the  particles  of  the  elements  that  chemi- 
cally combine  to  make  the  compound  substances  out  of  which  their 
organs  are  built  up  ! 


DEFINITIONS  AND  PROPERTIES  OF  MATTER.  33 

12.  Compressibility. — Compressibility  is  owing  to  porosity  of 
matter.     Diminution  of  volume  in  solids,  by  mechanical  means,  and 
by  loss  of  heat,  is  a  fact  well  known.    Even  columns  and  arches  of 
stone,    supporting  heavy  loads,  are  found  to   sensibly   diminish  by 
pressure  alone.     Metals  are  compressed  by  hammering.      Compressi- 
bility of  liquids  and  gases  will  be  alluded  to  under  the  head  of  hydro- 
statics and  pneumatics. 

13.  Expansibility. — Expansibility  and  contraction  of  all  bodies 
by   change  of   temperature    (heat    and   cold)    is  a  fact    sufficiently 
familiar.    Upon  this  property  of  matter  is  based  the  construction  of 
instruments  for  reading  changes  of  temperature.    This  subject  will  be 
again  referred  to  under  the  head  of  heat. 

'4 

IJj,.  Porosity— Physical  Pores.— The  facts  connected  with  the 
compressibility  of  matter  and  its  change  of  form  by  heat,  indicate 
that  the  ultimate  atoms  (assumed  to  be  unchangeable,  4)  are  not  in 
contact  (see  Fig.  1).  The  spaces  between  them  are  called  physical 
pores,  on  the  existence  of  which  depends  the  property  of  porosity. 
These  molecular  or  physical  pores  are  no  more  sensible  to  our  organs 
than  the  atoms  themselves,  and  are  permeable  only  to  light,  heat,  and 
electricity. 

Sensible  pores. — It  is  important  to  distinguish  the  molecular 
pores,  just  described,  from  those  sensible  openings  which  give  to 
certain  substances  the  property  generally  known  as  porosity.  The 
pores  of  organic  bodies,  as  of  wood,  skin,  and  tissues,  are  only  capil- 
lary openings,  or  canals  for  the  circulation  of  fluids. 

15.  Mobility. — By  mobility  is  meant  the  susceptibility  of  being 
set  in  motion.  Motion  is  recognized  only  by  comparing  the  moving 
body  with  some  other  body  at  rest.  Motion  and  rest  are  absolute  or 
relative.  For  instance,  a  person  walking  on  the  deck  of  a  moving 
ship  appears  to  change  his  place  in  reference  to  objects  on  the  deck, 
while  all  these  objects  are  in  motion  with  himself;  hence  his  motion 
is  only  relative ;  but  if  the  ship  is  at  rest,  then  his  motion  (referring 
to  these  objects  about  him)  is  absolute.  All  the  motion  on  the  earth's 
surface  is  relative,  for  the  reason  that  the  globe  itself  has  at  least  two 
motions;  one  around  its  axis  and  the  other  around  the  sun. 

Rest  is  absolute  when  the  body  really  occupies  the  same  point  in 
space,  and  relative  when  it  remains  the  same  apparent  distance  from 
surrounding  objects  which  are  not,  but  which  appear  to  be,  at  rest. 
For  example,  a  ship  sailing  up  a  river  at  the  rate  of  five  miles  an 

3 


34  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

hour,  while  the  water  flows  at  the  same  rate  in  the  opposite  direction, 
will  appear,  to  persons  on  its  deck  and  on  the  river-banks,  to  be  at  rest. 
Strictly,  there  is  no  absolute  rest.  There  is  not  an  atom  of  matter 
in  absolute  rest  throughout  the  boundless  expanse  of  space.  The 
earth,  and  all  the  other  planets,  and  all  their  satellites,  revolve  around 
their  own  centres  ;  and  the  moons  around  the  planets  ;  and  all  these 
around  the  sun;  and  the  sun,  with  all  these  planets  and  moons, 
together  with  other  solar  systems,  around  some  more  central  sun  ;  and, 
probably,  so  on,  systems  around  systems,  in  masses,  and  at  distances 
from  each  other,  and  with  velocities  that  astound  the  human  mind, 
and  exhibit  to  our  senses  the  infinite  power  and  wisdom  of  God. 

26.  Inertia.—  No  particle  of  matter,  in  a  state  of  rest,  possesses 
within  ^tself  the  power  of  putting  itself  in  motion  ;  or,  if  it  be  moving, 
to  bring  itself  to  a  state  of  rest.  A  body,  to  be  put  in  motion,  therefore, 
must  be  acted  upon  by  some  external  cause  ;  or,  conversely,  if  it  be  in 
motion,  it  cannot  cease  to  move  on  in  an  unchanging  direction  and 
with  an  unchanging  velocity,  without  the  application  of  some  opposing 
force.  This  passive  property  of  matter  is  termed  inertia. 

It  is  not  so  apparent  that  bodies  in  motion  will  not  of  themselves 
come  to  a  state  of  rest,  as  that,  being  at  rest,  they  will  not  of  themselves 
move.  This  is  because  the  motion  of  all  bodies  on  the  earth  is  con- 
tinually opposed  by  friction  and  the  resistance  of  the  atmosphere, 
which,  in  a  longer  or  shorter  time,  will  bring  them  to  a  state  of  rest, 
This  is  proved  by  the  fact  that  moving  bodies,  as  a  top  or  pendulum, 
will  not  so  readily  come  to  a  state  of  rest  in  a  vacuum  as  in  the  air  ; 
and  by  the  fact  that  the  same  body,  as,  for  example,  a  revolving  wheel 
with  large  or  small  bearings,  under  otherwise  the  same  circumstances, 
will  continue  longer  in  motion  the  more  friction  is  diminished.  The 
planets,  however,  not  being  influenced  by  these  or  other  obstructions, 
afford  examples,  and  the  only  examples,  of  constant  motion. 


Indestructibility.—  By  this  is  meant  that  matter  cannot  be 
annihilated.  Organized  bodies,  animal  and  vegetable,  can  be  reduced 
to  inorganic  substances;  and  compound  substances  to  simple  elements; 
and  simple  elements  to  various  forms,  from  solid  to  fluid,  and  from  fluid 
to  gaseous;  but  the  ultimate  atoms  cannot  be  changed  or  destroyed  (4). 
They  remain  forever  the  same.  For  example:  A  ton  of  coal,  by  the 
force  of  heat,  may  be  decomposed  and  reduced  to  a  few  pounds  of  ashes, 
and  the  balance  of  its  weight  to  large  volumes  of  smoke  and  invisible 
gas,  but  the  aggregate  weight  of  the  products  will  equal  the  weight  of 
the  coal.  Not  one  of  the  atoms  that  composed  the  coal  is  changed,  lost, 
or  destroyed.  This  is  the  case  through  all  the  ceaseless  motions  and 


DEFINITIONS  AND  PROPERTIES  OF  MATTER.  35 

infinite  variety  of  forms  which  matter,  acted  upon  by  the  forces  of 
nature,  is  made  to  pass. 

28.  Attraction.  —  There  are  several  kinds  of  attraction,  namely, 
gravitation,  cohesion,  adhesion,  capillarity,  affinity,  and  magnetic  and 
electric  attraction. 

Attraction  of  gravitation  is  that  force  or  form  of  attraction  by  which 
all  bodies,  at  sensible  distances,  tend  to  approach  each  other.  By  this 
force  every  atom  of  matter  in  the  universe  attracts  every  other  atom. 

Terrestrial  gravitation  is  that  manifestation  of  gravitation  which 
draws  all  bodies  on  the  earth  toward  its  centre. 

Attraction  which  takes  place  only  at  insensible  distances  is  termed 
molecular  attraction.  Of  this  there  are  four  kinds  : 

1st.  Cohesion,  which  binds  together  the  atoms  and  molecules  (4)  of 
matter.  It  is  this  force  which  binds  together  the  atoms  of  iron,  and 
holds  together  the  molecules  of  water,  to  form  bodies  or  masses. 

3d.  Adhesion,  which  exists  between  unlike  atoms  or  particles  of  mat- 
ter, when  in  simple  contact  with  each  other. 

3d.  Capillarity,  which  exists  between  liquids  and  tubes,  or  sensible 
pores  of  matter. 

4th.  Affinity,  which  unites  atoms  of  unlike  substances  into  com- 
pounds, possessing  new  and  distinct  properties  (1  and  2). 

Magnetic  and  electric  attraction  will  be  alluded  to  hereafter. 

19.  Varieties  of  motion.—  1st,  Translation,  or  direct  motion,  in 
which  all  the  points  of  a  body  move  parallel  to  each  other;  2d,  rota- 
tion, as  of  a  wheel  on  an  axis,  where  the  different  parts  of  a  body  move 
at  the  same  time  in  different  directions  ;  3d,  a  combination  of  transla- 
tion and  rotation,  as  in  the  motions  of  the  planets. 


Forces.—  By  'force,  as  used  in  mechanics,  is  meant  any  cause 
producing,  or  modifying,  motion.  In  this  sense  all  known  forces  have 
their  origin  in  three  causes  :  1st,  gravitation  (or  the  mutual  attraction 
of  bodies  for  each  other)  ;  2d,  the  unknown  cause  of  the  phenomena 
of  light,  heat,  and  electricity  ;  and,  3d,  life,  or  the  mysterious  agency 
producing  motion  of  animals. 

21.  Momentum.  —  The  momentum  of  a  body  is  its  amount  of 
motion,  or  its  tendency  to  continue  in  motion,  and  it  is  equal  to  the 
mass  or  weight  of  the  body  multiplied  by  its  velocity. 

22.  Cohesion  and  repulsion.  —  These  are  two  opposing  princi- 
ples or  forces  inherent  in  the  atoms  and  molecules  of  matter,  termed 


36  MATETR,  FORCE,  MOTION,  AND  MECHANICS. 

molecular  forces.  These  are  either  attractive  or  repulsive,  drawing  the 
particles  of  bodies  toward  each  other,  or  tending  to  separate  them. 
Though  this  attractive  force  (termed  cohesion)  draws  the  particles  to- 
ward each  other,  it  is  not  supposed  they  come  into  actual  contact,  being 
prevented  from  doing  so  by  their  mutual  repulsion.  It  is  this  that  ac- 
counts for  the  insensible  or  physical  pores  (14),  existing  between  the 
atoms  and  molecules  of  all  matter. 

28.  Relation  of  cohesion  and  repulsion  in  the  three 
states  of  matter.  —  Matter  exists  in  three  states  :  the  solid,  liquid, 
and  gaseous.  *  In  solids  the  attractive  force  greatly  overpowers  the  re- 
pulsive, holding  the  particles  in  a  relatively  fixed  position  at  certain 
distances  from  each  other.  Heat  will  increase  the  power  of  repulsion, 
and  cold  (decrease  of  heat)  will  diminish  it  ;  hence,  by  varying  the 
heat  of  a  body  its  size  can.  be  sensibly  varied,  which  is  the  result  of 
altering  the  distance,  and  size  of  the  pores,  between  the  particles. 

In  liquids  or  inelastic  fluids  these  two  forces  are  in  perfect  equilib- 
rium, which  leaves  the  particles  to  move  with  perfect  freedom  among 
themselves  (88  and  90). 

In  gases  or  elastic  fluids  the  repulsive  force  holds  sway,  which  causes 
the  body  to  dilate,  unless  confined  by  external  force  (91).  Liquids  and 
gases  as  well  as  solids  are  contracted  and  expanded  by  variations  of 
heat. 


structure  in  solids  is  meant  relative  disposition  of 
their  atoms  and  molecules  or  their  groups.  This  structure  may  be 
either  symmetrical  or  regular,  as  in  living  beings  arid  crystals;  or 
amorphous,  as  in  most  rocks  and  many  other  substances. 

There  is  a  formative  force  or  principle  pervading  or  inherent  in  all 
matter,  disposing  or  arranging  its  atoms  and  molecules  and  their  groups 
in  definite  forms.  In  the  organic  —  vegetable  and  animal  —  kingdoms 
this  force  or  principle  is  termed  vitality  ;  and  forms  thus  produced  are 
mostly  bounded  by  curved  lines  and  surfaces.  In  the  inorganic  or  life- 
less world,  different  formative  forces  or  principles  govern  the  arrange- 
ment of  particles;  forming,  under  favorable  circumstances,  bodies 
which  are  angular  and  bounded  by  plane  faces.  Such  bodies  are 
termed  crystals  ;  and  their  geometrical  forms,  thus  produced,  are  anal- 
ogous to  the  more  complicated  forms  found  in  animal  and  vegetable 
life.  The  formative  principle  in  inorganic  bodies  is  easily  or  often 
interrupted,  which  accounts  for  so  many  irregular  forms  of  crystals. 


DEFINITIONS  AND  PROPERTIES  OF  MATTER.  37 


THE   CHARACTERISTIC   PROPERTIES   OF   SOLIDS: 

Some  of  these  depend  upon  the  atomic  structure  of  the  material  ; 
as,  Crystalline  form,  Elasticity,  Resistance,  and  Hardness  ;  and  others 
upon  a  permanent  change  in  the  arrangement  of  the  molecules  ;  as, 
Malleability,  Ductility,  Temper,  etc. 

25.  Crystalline  form.  —  The  molecules  of  certain  substances, 
when  left  free  to  move  among  themselves,  by  means  either  of  solution, 
fusion,  sublimation,  or  electrical  or  chemical  decomposition,  will  be 
acted  upon  by  the  force  of  crystallogenic  attraction,  and  thereby  be 
united  into  solids  or  masses  of  certain  definite  and  uniform  shapes, 
termed  crystals  —  each  substance  yielding  its  own  peculiar  form. 


.  Elasticity.  —  This  is  that  property  of  matter  which  disposes 
it  to  resume  its  original  form  or  shape  after  having  been  bent  or  com- 
pressed by  some  external  force.  It  is  not  so  much  a  distinct  property 
of  matter  as  it  is  a  phenomenon  of  attractive  and  repulsive  forces.  Dif- 
ferent bodies  possess  this  property  in  very  different  degrees. 

When  elasticity  takes  place  in  the  direction  of  the  length  of  the 
body,  as  a  wire,  it  is  termed  elasticity  of  tension  and  compression  ; 
when  in  a  direction  transversely  to  the  body,  as  in  the  case  of  a  bent 
beam,  it  is  termed  elasticity  of  flexure  ;  and  when  a  body  is  twisted  by 
a  force  applied  at  one  end  while  the  other  extremity  is  fixed,  it  is  called 
elasticity  of  torsion. 

The  limit  of  elasticity  of  any  body  (whatever  be  its  substance)  is 
reached  when,  if  the  applied  force  ceases  to  act,  the  body  will  fail  to 
come  back  to  its  original  position.  If  the  force  acts  beyond  the  limit 
of  elasticity,  the  molecules  are  forced  into  new  relations  with  each 
other,  and  the  body  is  said  to  have  been  forced. 

27.  Resistance   to   fracture,    or  tenacity,  is  that   property 
which  resists  separation  of  the  particles  longitudinally  or  transversely, 
and  gives  strength  to  materials,  and  depends  upon  the  force  of  cohesive 
attraction,  which  varies  greatly  in  different  substances,  according  to 
the  nature  of  the  atoms  or  molecules  composing  them. 

28.  Hardness  is  that  property  in  virtue  of  which  the  particles  of 
bodies  resist  impression,  or  the  action   of  any   force  that  tends  to 
change  their  form.     Hence,   a  body  whose  particles  can  be  easily 
changed  in  their  relative  position,  by  slight  forces,  is  said  to  be  soft  ; 
therefore,  softness   is  the  opposite  of  hardness.      Hardness   does   not 
imply  density  ;  for  example,  lead  is  more  dense  but  softer  than  iron. 


38  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

29.  Malleability  is  the  property  of  being  wrought  under  the 
hammer,  and  belongs  to  many  of  the  metals  in  an  eminent  degree  ; 
and  upon  it  largely  depends  their  utility.  The  most  malleable,  in  order 
of  softness,  are  lead,  tin,  gold,  zinc,  silver,  copper,  platinum,  and  iron. 
But  gold  may  be  beaten  thinner  than  any  other  metal.  It  can  be 
hammered  so  thin  that  the  thickness  of  a  million  leaves  will  be  less 
than  an  inch. 

SO.  Ductility  is  the  property  in  virtue  of  which  a  substance 
admits  of  being  drawn  into  wire,  which  is  not  altogether  unlike 
malleability;  yet  there  is  a  marked  difference,  as  shown  by  the  fact 
that  the  most  malleable  are  not  the  most  ductile  substances.  Tin  and 
lead  are  highly  malleable  but  not  ductile,  as  they  cannot  be  drawn  into 
fine  wire  ;  while  gold  is  both  very  malleable  and  ductile  ;  having  been 
drawn  into  wire  so  fine  that  one  ounce  of  it  would  extend  fifty  miles. 

31.  Flexibility  and  pliability  are  those  properties  which  per- 
mit considerable  motion  between  the  particles  of  a  body  without  their 
passing  beyond  the  reach  of  their  power  of  cohesive  attraction.  Bodies 
of  this  kind  are  not  easily  broken.  These  properties  differ  from  elas- 
ticity in  that,  that  elastic  bodies,  within  their  limits  of  elasticity,  re- 
sume their  original  form,  while  with  flexible  bodies  the  original  form 
is  not  resumed. 


Brittleness  is  the  property  which  renders  bodies  easily  broken 
into  fragments.  It  is  the  characteristic  of  most  hard  substances.  In 
a  brittle  body  the  attractive  force  between  the  atoms  or  molecules 
exists  within  such  narrow  limits,  that  a  very  slight  change  of  position, 
or  increase  of  distance  among  them,  is  sufficient  to  overcome  it,  and 
the  body  breaks. 

;» 

33.  Hardening  and  annealing.  —  Some  bodies,  as  steel  and 
iron,  by  being  brought  to  a  high  temperature  and  then  suddenly  cooled, 
by  plunging  into  cold  water,  oil,  or  mercury,  will  become  very  hard. 
This  is  called  tempering,  and  the  hardness  is  supposed  to  be  caused 
by  thus  producing  a  slight  change  in  the  relative  position  of  the  atoms 
or  molecules  of  the  substance.  Some  substances,  as  bronze,  are  hard- 
ened by  being  hammered,  and  others,  as  zinc  and  iron,  by  being  rolled. 

It  is  singular,  however,  that  heating  and  sudden  cooling  should 
harden  some  substances,  as  steel,  while  other  substances,  as  copper, 
will  be  softened  by  the  same  process.  This,  probably,  is  owing  to  the 
fact  that  heat  and  cold  do  not  produce  the  same  changes  in  the  relative 
position  of  the  atoms  in  the  one  substance  that  they  do  in  the  other. 


ATTRACTION. 


39 


Welding  is  the  process  of  uniting  the  atoms  of  substances, 
as  iron  to  iron  or  iron  to  steel,  by  cohesive  attraction,  which  is  accom- 
plished by  hammering  them  together  when  at  a  high  temperature, 
which  brings  the  particles  so  close  together  that  they  are  brought 
within  the  reach  of  their  cohesive  force. 


FIG.  1. 


CHAPTER    III. 

ATTRACTION. 
Molecular  Attraction. 

35.  Figure   1. — Interstices  between  atoms   and   mole- 
cules of  matter. — The  three  different  sized  balls  or  spheres  in  the 
diagram,  may  represent  bodies  of  matter,  as 

cannon-balls,  bullets,  and  shot,  or  as  apples, 
plums,  and  currants  ;  or  they  may  represent 
atoms  or  molecules  (4)  of  matter,  which  vary 
in  size,  as  those  of  water,  salt,  and  sugar. 

If  a  vessel  be  filled  as  full  as  possible  with 
water,  considerable  salt  may  be  put  into  the 
vessel  without  disturbing  the  water,  and  then 
a  quantity  of  sugar  can  be  introduced  with- 
out displacing  the  water,  which  may  be  ac- 
counted for  by  supposing  the  molecules  of 
these  various  substances  to  be  spherical  and  of  different  sizes,  and, 
probably,  not  in  absolute  contact  (22),  as  shown  in  the  figure. 

36.  Figure  2.— Cohesive  attraction. — Cohesion  is  the  force  of 
attraction  which  holds  atoms  and  molecules  of  the  same  bodies  to- 
gether ;  as,  for  example,  a  mass  of  stone,  iron,  or  wood. 

This  figure  represents  two  hemi- 
spheres of  lead,  with  their  flat  sur- 
faces made  very  smooth,  and  joined 
together  by  firmly  rubbing  one 
against  the  other. 

If  cords  be  fastened  to  the  side 
projections  and  an  effort  made  to 
separate  these  hemispheres,  it  will 
be  found  that  more  than  fifteen 
pounds  of  force  to  the  square  inch 
of  the  surface  between  them  (which 


FIG.  2. 


40  MATTER,  FORGE,  MOTION,  AND  MECHANICS. 

represents  the  atmospheric  pressure)  is  required  to  draw  them  asunder  ; 
thus  proving  they  are  held  together  by  cohesive  attraction.  Other 
smooth  substances  present  the  same  phenomena,  but  with,  different 
degrees  of  intensity.  Cohesion  can  be  shown  independent  of  atmo- 
spheric pressure,  by  separating  the  hemispheres  in  the  vacuum  of  an 
air-pump. 


.  Figure  3.—  Adhesive  attraction.—  This  force  of  attraction 
is  that  which  holds  the  molecules  of  dissimilar  bodies  together,  and 

is  termed  adhesion.  It  is  adhesion 
that  holds  wood,  glue,  and  paint 
together,  and  causes  liquids  to  ad- 
here to  solids. 

Let  L  be  a  disk  of  glass  or  metal, 
counterpoised  by  a  scale-pan,  and 
so  adjusted  that  the  disk  will  just 
touch  the  surface  of  the  liquid  ; 
then  place  in  the  scale-pan  just  suf- 
ficient weight  to  separate  the  disk 
from  the  liquid,  and  this  will  indi- 
cate the  measure  of  adhesion  be- 
tween them. 

The  experiment  will  also  indicate  the  force  of  cohesion  among  the 
particles  of  liquid,  which  is  somewhat  less  than  the  adhesion  between 
the  liquid  and  solid  ;  for,  were  it  not  less,  then  none  of  the  liquid 
would  adhere  to  the  disk  and  thus  be  separated  from  itself,  and  the  disk 
would  come  up  dry.  The  force  of  cohesion  is  not  the  same  in  all  liquids  ; 
that  of  alcohol  and  turpentine  being  but  little  more  than  half  as  intense 
as  that  of  water. 

38.  Figure  4.  —  A  few  phenomena  of  capillarity.  —  Capil- 
lary forces  are  molecular  forces  exerted  between  liquids  and  tubes,  or 
liquids  and  sensible  pores  of  bodies  (14). 

If  tubes  of  small  bore,  open  at  both  ends,  are  placed  vertically  in 
water,  the  liquid  will  rise  both  in  the  tubes  and  on  the  outside,  as 
shown  at  H  and  J  ;  rising  higher  within  as  the  tubes  are  smaller,  as 
seen  at  J.  If  the  tube  is  over  half  an  inch  in  diameter  the  effect  is 
hardly  observable.  If  mercury  is  employed  (which  does  not  wet  the 
glass)  there  is  a  depression  of  the  surface  of  the  liquid,  both  within 
and  without  the  tube,  as  exhibited  at  Y  ;  and  this  becomes  greater 
as  the  tubes  are  smaller.  In  a  greased  tube  water  is  similarly  de- 
pressed. 

These  phenomena  are  independent  of  atmospheric  pressure  ;  taking 


ATTRACTION. 


41 


place  equally  in  a  vacuum  or  compressed  air.     They  vary,  however,  with 
the  material  of  the  tube  and  with  the  nature  of  the  liquid. 

The  attraction  and  repulsion  observed  between  light  bodies  floating 
on  liquids  is  due  to  capillarity.  The  floating  bodies  are  drawn  near  to 
each  other,  either  when  both  are  or  are  not  moistened,  as  at  L  and  P, 
and  repelled  if  the  liquid  wets  only  one  of  them,  as^at  R.  At  L,  both 

FIG.  4. 


balls  being  moistened,  the  liquid  rises  (by  capillarity)  higher  between 
than  on  the  outside  of  them,  which  acts  as  a  loaded  cord  to  draw  them 
together;  while  at  P,  both  balls  being  dry,  the  water  is  depressed  lower 
between  than  outside  of  them,  which  causes  them  to  be  crowded  to- 
gether. At  R,  one  ball  being  wet  and  the  other  dry,  causes  the  water  to 
rise  around  one  and  to  be  depressed  around  the  other,  which  effects, 
combined,  build  up  an  inclined  plane  between  them,  and  thus  they  are 
kept  apart,  as  shown  in  the  figure. 


Gravitation. 

39.  Gravitation. — The  attraction  of  cohesion,  as  has  been  shown, 
unites  particles  of  matter  into  masses  or  bodies,  and  the  attraction  of 
gravitation  tends  to  draw  these  masses  together  to  form  bodies  of 
greater  dimensions. 

Weight. — The  fall  of  a  body  to  the  earth,  and  its  downward  pres- 
sure upon  the  earth's  surface,  are  due  to  the  force  of  gravity  ;  and  the 
amount  of  this  pressure  is  called  the  weight  of  the  body. 

Jj,0.  Figure  5. — Centre  of  gravity  of  bodies. — The  centre 
of  gravity  of  a  body  is  that  point  through  which  the  direction  of  the 
weight  always  passes,  and  this  point  coincides  with  the  centre  of  in- 
ertia. The  figure  shows  that  when  two  or  more  bodies  are  connected 


42  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

FIG.  5.  together,    they  may  be   re- 

garded as  one  body,  having 
but  one  centre  of  gravity.  If 
the  fulcrum,  T,  supports  the 
centre  of  gravity  of  the  two 
bodies,  they  will  remain  at 
rest ;  and  this  point,  if  the 
bodies  are  of  equal  weight, 
will  be  in  the  middle  of  the 
line  which  unites  them 
(measuring  from  the  separate 
centres  of  gravity) ;  but  if  they  be  of  unequal  weight,  the  centre  of 
gravity  will  be  as  much  nearer  the  heavier  body,  as  the  heavier  exceeds 
the  lighter  one  in  weight. 

A  prop  that  supports  the  centre  of  gravity  supports  the  whole  body, 
which  may  be  applied  directly  at  the  centre  of  gravity,  or  immediately 
above  or  beloio  it,  on  the  line  that  points  to  the  earth's  centre  of 
gravity. 

A  body  is  in  a  state  of  equilibrium  when  its  weight  is  completely 
counteracted  by  supporting  the  centre  of  gravity. 

There  are  three  kinds  of  equilibrium;  stable,  unstable,  and  neutral. 
A  body  is  in  stable  equilibrium  when,  on  being  slightly  disturbed 
from  its  state  of  rest,  it  tends,  of  itself,  to  return  to  that  state.    A  rock- 
ing-chair is  a  case  of  this  kind. 

A  body  is  in  unstable  equilibrium  when,  on  being  slightly  disturbed 
from  its  state  of  rest,  it  does  not  tend  to  return  to  that  state,  but  con- 
tinues to  depart  from  that  state  more  and  more. 

A  body  is  in  a  neutral  equilibrium  when,  on  being  slightly  disturbed, 
it  has  no  tendency  either  to  return  to  its  former  state  or  to  depart 
further  from  it.  The  last  two  kinds  of  equilibrium  are  illustrated  by 
Fig.  7  (43.) 

Jj,l.  Figure  6.— Method  of  finding  the  centre  of  gravity 

of  irregular  shaped  bodies.  Let  the  ob- 
ject be  freely  suspended  from  some  point, 
as  H,  and  the  centre  of  gravity  will  fall 
into  the  vertical  line  HV  (marked  by 
plummet  and  line).  If  now  the  body  be 
freely  suspended  from  some  other  point, 
as  T,  the  centre  of  gravity  will  again 
fall  into  the  vertical  line,  which  (being 
marked  by  the  plumb-line  TL),  will  be 
found  to  cross  the  line  HV ;  therefore, 


ATTRACTION. 


43 


FIG.  7. 


the  centre  of  gravity  will  be  at  the  intersection  of  these  two  lines,  for 
it  cannot  be  in  both  lines  at  any  other  point.    . 

42.  Figure  7.— Neutral  and  unstable  equilibrium  illus- 
trated with  a  wheel  or  ball  on  a  horizontal  and  inclined  plane. 

If  the  plumb-line  from  the  centre  of  the  wheel  or  ball  (which  is  the 
centre  of  gravity)  be  drawn,  it  will 
pass  through  the  base  or  point  on 
which  it  rests  on  the  horizontal 
plane  N,  which  supports  the  body 
from  moving.  This  will  be  the  case 
whichever  side  up  the  ball  may  be, 
or  on  whatever  point  of  the  plane  it 
may  be  placed,  affording  an  illustra- 
tion of  neutral  equilibrium. 

If  the  horizontal  plane  be  removed, 
and  the  ball  allowed  to  bear  upon  the 
inclined  plane  T,  the  point  of  contact 
is  back,  or  at  one  side  of  the  plumb- 
line,  which  deprives  the  centre  of  grav- 
ity of  its  vertical  support ;  therefore,  in 

search  of  support  it  will  begin  to  fall,  and  continue  to  descend  in  the 
direction  of  the  dotted  line  L,  which  is  parallel  to  the  inclined  plane. 

This  figure  also  shows  that  the  reason  why  a  wheel  or  ball  is  so  easily 
moved  over  a  horizontal  plane  is,  because  the  centre  of  gravity  is  not 
elevated  by  the  movement.  If  the  ball  be  rolled  along  the  plane  N,  the 
centre  of  gravity  will  pass  along  the  dotted  horizontal  line  Y,  which 
neither  rises  nor  falls. 

43  •  Figure  8.— Stability  of  bodies  depends  upon  the 
position  of  the  centre  of  grav- 
ity.— Suppose  the  diagram  to  repre-  IG' 
sent  a  cube  of  wood,  iron,  or  stone, 
with  its  centre  of  gravity  indicated 
by  the  dot  at  the  centre. 

Place  one  foot  of  the  dividers  at 
the  lower  left  hand  corner  of  the 
cube  and  the  other  at  the  centre 
of  gravity,  and  draw  the  curved 
dotted  line,  and  with  a  rule  draw  the 
straight  dotted  horizontal  line;  and 
the  distance  between  these  two  lines, 
at  their  intersection  with  the  surface  of  the  block,  will  equal  the  vertical 


44 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


FIG.  9. 


distance  that  the  centre  of  gravity,  or  the  weight  of  the  body,  must  be 
elevated  in  order  to  overturn  the  block. 

44-  Figure  9.— Relative  stability  of  cubes  and  pyra- 
mids.— Let  the  altitude  and  base  of 
the  pyramid  be  the  same  as  those  of 
the  above  cube,  with  the  centre  of 
gravity  indicated  by  a  dot,  which  is  at 
one- third  of  the  distance  from  the  base 
to  the  apex.  The  space  L,  it  will  be 
seen,  between  the  two  dotted  lines,  is 
much  greater  than  in  the  previous 
figure ;  showing  that,  as  the  centre  of 
gravity  in  the  pyramid  is  nearer  the 
base  than  in  the  cube,  the  weight  re- 
quires to  be  elevated  a  greater  vertical 
distance  in  order  to  be  passed  over  the 

edge  of  the  base ;  hence  the  pyramid  has  greater  stability  than  the  cube. 
The  stability  of  any  body,  at  rest,  of  given  bulk  and  weight,  depends 

upon  how  far  the  centre  of  gravity  must  be  elevated  in  order  to  pass 

it  over  the  edge  of  its  base  nearest  to  the  vertical  line  passing  through 

its  centre  of  gravity. 

45.  Figure  10.— Centre  of  gravity  of  vehicles.— Suppose 
the  vehicle,  freighted  with  lead,  to  be  moving  across  an  inclined  plane, 

and  the  centre  of  gravity  to  be  at  L ; 
then  the  line  of  direction,  shown  by 
the  arrow  on  the  right,  would  fall 
within  the  base ;  that  is,  between  the 
wheels  of  the  vehicle,  in  which  case 
it  would  not  be  overturned.  •  If  the 
vehicle  were  loaded  with  such  mate- 
rial as  would  bring  the  centre  of 
gravity  at  N,  it  would  not  be  over- 
turned, as  the  line  of  direction,  shown 
by  the  middle  arrow,  still  falls  be- 
tween the  wheels  ;  but  if  the  wagon 
were  so  freighted  as  to  bring  the 
centre  of  gravity  at  T,  it  would  be 
overturned,  because  then  the  line  of 
direction  would  fall  without  the 
wheels  or  base,  as  shown  by  the  arrow 
on  the  left. 


FIG.  10. 


ATTRACTION. 


45 


PIG.  11. 


.  Figure  1 1  .—Centre  of  grav- 
ity in  man. — The  centre  of  gravity 
in  man  being  between  his  hips,  the 
line  of  direction,  if  he  stands  erect,  will 
fall  within  his  base,  that  is,  between  his 
feet.  But  if  he  carries  a  burden  he  will 
lean  in  the  opposite  direction  from  it, 
in  order  to  bring  the  resultant  centre  of 
gravity  of  himself  and  burden  into  the 
vertical  line  passing  down  between  his 
feet,  as  shown  by  the  dotted  arrow; 
otherwise  the  line  of  direction  would 
fall  without  the  base,  that  is,  outside  of 
his  feet,  and  it  would  be  impossible  to 
prevent  himself  from  falling. 


47.  Figure  12.— Law  of  inten- 
sity of  gravity. — The  force  of  grav- 
ity varies  directly  in  proportion  to  the 
quantity  of  matter  contained  in  bodies, 
and  inversely  as  the  square  of  the  dis- 
tance between  them,  measuring  from  their  centres  of  gravity. 

Let  the  diverging  lines,  in  the  diagram,  represent  lines  of  attraction, 
then  the  small  parallelogram  formed  between  FlG 

them  at  the  earth's  surface  may  represent  the 
force  of  gravity  at  this  point,  which  equals  1 ; 
and  its  distance  from  the  centre  of  the  earth 
equals  1.  If  further  on,  at  a  distance  equal 
to  2,  another  parallelogram  be  constructed  be- 
tween these  diverging  lines,  it  will  be  four 
times  as  large  as  the  small  one,  shown  by  the 
dotted  division  lines;  and  if  at  a  distance 
equal  to  3  a  parallelogram  be  drawn,  it  will  be 
nine  times  as  large,  and  so  on.  Now,  as  there 
is  only  a  given  amount  of  attraction  between 
these  diverging  lines,  it  follows  that  its  inten- 
sity diminishes  as  the  space  between  these  lines 
increases  ;  and,  as  1,  4,  and  9  are,  respectively, 
the  squares  of  1,  2,  and  3,  this  space  increases 
in  the  same  ratio  as  the  square  of  the  dis- 
tance (from  the  earth's  centre  of  gravity)  in- 
creases. 

This  law  is  stated  in  a  tabular  form  thus : 


46  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


Distances  I  1  1 

2 

|3 

1  4   \   I 

»   1   « 

V     1 

8 

1    »     I 

10 

|   11   |   12 

etc. 

Intensity  of  attraction  |  1  | 

4 

g 

\  Is  \  * 

5    |    :,V 

4V! 

6T 

1  -sLr  | 

T^IT 

1  T^T  |  TiT 

etc. 

Conditions  Affecting  Terrestrial  Gravity. 

Jj,8.  Gravity  affected  by  altitude. — In  accordance  with  this 
law  of  attraction,  if  a  body  at  4,000  miles  from  the  centre  of  the  earth 
(which  would  be  at  its  surface)  weighs  1  pound,  at  8,000  miles  (that 
is,  4,000  from  the  surface)  it  would  weigh  J  of  a  pound,  and  at  12,000 
miles  \  of  a  pound,  and  so  on.  Hence,  bodies  weigh  slightly  less  on 
mountains  than  at  the  level  of  the  sea. 

4,9.  Gravity  affected  by  depression  below  level  of  the 
sea. — In  passing  from  the  surface  to  the  centre  of  the  earth  the 
weight  decreases  also — not,  however,  as  the  square  of  the  distance,  but 
directly  as  the  distance  increases ;  for,  at  the  centre  of  the  earth,  the 
weight  or  gravity  of  a  body  would  be  nothing,  as  the  distance  from  the 
centre  of  gravity  is  nothing.  And,  in  this  case,  the  body  would  be 
attracted  by  the  earth  equally  in  all  directions.  A  body,  therefore,  that 
weighs  a  pound  at"  the  surface,  would  weigh  only  half  a  pound  if  it 
could  be  placed  half  way  from  the  surface  to  the  centre  of  the  eartlu 
Hence,  bodies  will  slightly  decrease  in  weight  as  they  are  placed  in 
deep  excavations  below  the  level  of  the  sea. 

50.  Gravity  affected  by  shape  of  the  earth. — Owing  to  the 
flattening  of  the  earth  at  the  poles,  a  body  at  the  equator  will  be  13 
miles  further  from  the  centre  of  gravity  of  the  earth  than  when  it  is  at 
either  pole.    Hence,  it  will  weigh  less  at  the  equator  than  at  the  poles  ; 
and,  for  this  cause  alone,  the  difference  in  weight  would  be  about  ^-g* 
But  it  is  found  that  the  actual  difference'  is  Ttr  of  the  equatorial  weight: 
that  is,  a  body  weighing,  at  the  equator,  194  pounds,  would  weigh,  at 
the  poles,  195  pounds ;  showing  that  this  great  difference  must  be  ac- 
counted for  by  some  other  cause ;  which  is  the  centrifugal  force  result- 
ing from  the  rotation  of  the  earth. 

51.  Gravity  affected  by  the  earth's  rotation.— The  centrif- 
ugal force  (caused  by  the  rotation  of  the  earth  on  its  axis),  which  is. 
nothing  at  the  poles  and  regularly  increases  toward  the  equator,  where 
it  is  greatest,  in  the  same  ratio  diminishes  the  weight  of  bodies  on  the 
earth's  surface.    If  the  earth  were  to  revolve  seventeen  times  faster 
than  it  now  does  (or  once  in  1  h.  24  m.  25  s.),  the  centrifugal  force 
would  balance  the  force  of  gravity,  and  bodies  at  the  equator  would 
have  no  sensible  weight ;  while  at  the  poles  the  weight  of  bodies  would 
remain  the  same.     If  the  earth  were  to  revolve  in  less  time  than  about 


MOTION  AND  FORCE.  47 

1  h.  24  m.,  the  oceans  would  be  thrown  off  at  the  equator,  like  water 
from  a  grindstone,  and  loose  bodies  would  fly  into  space  above  the 
earth's  surface. 

52.  Earth  drawn  toward  falling  bodies.— A  body  falling 
through  space  to  the  earth  also  draws  the  earth  through  space  toward 
itself.    The  mass  of  the  earth,  however,  being  so  much  greater  than 
any  of  its  detached  bodies,  and  the  relative  distances  that  the  earth  and 
the  detached  bodies  move  being  inversely  as  their  masses,  of  course, 
the  earth's  motion  would  be  incalculably  small,  but  mathematically  a 
fact. 

53.  Direction  of  gravity. — The  direction  in  which  gravity  acts 
corresponds  to  straight  lines,  drawn  from  the  earth's  centre  of  gravity, 
and  perpendicular  to  the  earth's  surface.     The  two  small  balls  and  lines 
on  the  right  of  the  diagram  (Fig.  12)  may  represent  two  plumb-lines, 
which,  by  gravity,  are  attracted  toward  the  centre  of  gravity  of  the 
earth,  as  seen  by  the  continued  dotted  lines ;  thus  showing  that  plumb- 
lines,  though  sensibly  parallel,  are  not  mathematically  so  ;  and  hence, 
proving  that  the  walls  of  a  building,  for  instance,  laid  up  by  plumb- 
lines,  are  not  exactly  vertically  parallel  with  each  other. 

Up  and  down,  relative  terms.— As  the  law  of  direction  of 
gravity  holds  good  on  all  sides  of  the  earth  alike,  it  shows  that  up  and 
down  are  only  relative  terms — up  meaning  away  from  the  earth,  and 
down  signifying  toward  it ;  so  that  the  direction  up  to  us  would  be 
down  to  our  antipodes ;  or  antipodes  pointing  one  up  and  one  down 
would  point  in  the  same  absolute  direction. 


CHAPTEK   IY. 

MOTION  AND  FORCE. 

Motion  and  force.— There  are  three  varieties  of  motion : 
translation,  or  direct  motion  ;  motion  of  rotation ;  and  a  combination 
of  translation  and  rotation  (see  15  and  19).  Besides  these  there  are 
uniform  motion;  accelerated  motion  ;  and  retarded  motion. 

A  body  has  uniform  motion  when  it  moves  over  equal  spaces  in  equal 
times. 

A  body  has  uniformly  accelerated  motion  when  its  velocity  increases 
by  a  constant  quantity  in  a  given  time. 


48  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

A  body  has  uniformly  retarded  motion  when  its  velocity  diminishes 
by  a  constant  quantity  in  a  given  time. 

The  increase  of  velocity  in  a  second  is  called  the  acceleration ;  and 
the  decrease  in  a  second  the  retardation. 

Force. — For  definition  and  origin  of  force,  see  20.  In  determining 
a  force  there  must  be  taken  into  consideration :  1st,  the  point  of  appli- 
cation ;  3d,  the  direction ;  3d,  the  intensity  or  energy  with  which  the 
force  acts. 

Forces  are  represented  by  lines ;  and  any  given  length  of  line  may 
be  taken  as  the  unit  of  force  ;  hence,  the  direction  of  a  line  will  repre- 
sent the  direction  in  which  the  force  acts ;  and  its  length,  the  magni- 
tude or  intensity  of  the  force. 

Statics  is  the  science  of  equilibrium;  and  dynamics  treats  of  the 
motions  which  forces  produce. 

Equal  forces  acting  in  opposite  directions,  the  body  upon  which 
they  act,  as  also  the  forces  themselves,  are  said  to  be  in  equilibrium. 

The  direction  in  which  a  force  is  applied  determines  the  direction  in 
which  the  body  receiving  the  force  will  move,  or  of  the  resultant 
pressure  if  the  body  is  not  free  to  move. 

Measure  of  forces. — The  following  propositions  will  express  the 
effects  of  different  forces : 

1.  Two  constant  forces  are  to  each  other  as  the  masses  to  which,  in 
equal  times,  they  impart  equal  velocities. 

2.  Two  constant  forces  are  to  each  other  as  the  velocities  which  they 
impress,  during  the  same  time,  upon  two  equal  masses. 

3.  Two   constant  forces  are  to  each  other  as  the  products  of  the 
masses,  by  the  velocities  which  they  impart  to  these  masses  in  the  same 
time. 

4.  The  measure  of  a  force  is  the  product  of  the  mass  moved  by  the 
acceleration,  or  velocity,  imparted  in  a  unit  of  time. 

The  momentum  of  a  body  is  equal  to  its  mass  or  weight  multiplied 
by  its  velocity. 

55.  Figure  13.— Laws  of  falling  and  rising  bodies.— The 

main  line  in  the  diagram,  for  convenience,  is  divided  into  four  equal 
parts,  H,  N,  L,  and  R,  of  16  feet  each,  to  represent  the  track  of  a  fall- 
ing or  rising  body  during  two  seconds  of  time. 

It  has  been  found  by  experiment  that  a  body  starting  from  a  state 
of  rest  will  fall  16  feet  the  first  second,  and  that  its  velocity  at  the 
starting  point  is  nothing,  and  at  the  end  of  the  second  it  is  equal  to  32 
feet  per  second,  showing  that  the  average  is  just  half  the  accelerated 


MOTION  AND  FORCE. 


49 


velocity.  At  the  end  of  the  next  second  it  will  have  ac- 
quired another  acceleration  of  32  feet,  which,  added  to  the 
first  acceleration,  makes  64  feet. 

The  body  S,  therefore,  would  fall  through  the  space  H 
the  first  second,  and  its  acquired  velocity  of  32  feet  would 
carry  it  the  next  second  over  the  lines  N  and  L,  and  the 
force  of  gravity  would  (of  itself)  carry  it  over  the  line  K ; 
hence,  it  would  fall  during  the  1st  second  16  feet,  and 
during  the  3d  second  32  feet  +  16  feet  =  48  feet;  and 
during  the  3d  second  64  feet  +  16  feet  =  80  feet,  and 
during  the  4th  second  96  feet  +  16  feet  =  112  feet ;  and 
so  on. 

Or,  thus  stated,  it  would  fall  during  the 


FIG.  13. 


1st  second  16  feet 

2d        «       32  +  16 

3d        "       32  +  32  +  16 

4th      "       32  +  32  +  32  + 


=  16  =16  feet. 

=  32  +  16  =    48    " 

=  64  +  16  =     80    " 

16  =  96  +  16  =  112    « 


Or,  by  adding,  we  have  the  space  passed  over  in  the 


1st  second 
2d      « 


-  16  feet. 

-  64    " 


3d  second  - 
4th     " 


-  144  feet. 

-  256    « 


It  will  be  seen  that  these  spaces  are  to  each  other  as  the 
squares  of  the  time ;  that  is, 

I2  is  to  22  as  16  is  to  64. 
I2  is  to  3a  as  16  is  to  144. 
I2  is  to  42  as  16  is  to  256,  or, 

22   is  to   32   as     64   is  to   144. 

32   is  to   42   as  144  is  to  256;  and  so  on. 

Or,  in  tabular  form : 


The  intervals  being 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

etc. 

The  spaces  described 
each  interval 

1 

3 

5 

7 

9 

11 

13 

15 

17 

19 

And  the  whole  space 
will  be 

1 

4 

g 

16 

25 

36 

49 

64 

81 

100 

As  the  motion  of  a  body  is  uniformly  accelerated  when 
falling  to  the  earth,  so  it  is  uniformly  retarded  when  rising 
from  the  earth,  passing  over  spaces  decreasing  each  interval 
as  the  square  of  the  time. 


50 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


Fig.  14. 


To  find  the  velocity  of  a  falling  body  at  the  end 
of  any  second,  multiply  32  feet  by  the  number  of 
seconds  it  has  been  falling. 

To  find  the  velocity  of  a  rising  body  at  any  par- 
ticular second,  multiply  the  seconds  it  has  been 
rising  by  32  feet,  and  subtract  this  from  the  velo- 
city it  had  at  starting. 

Bodies  also  acquire  the  same  velocity  in  falling 
the  same  perpendicular  distance  from  a  state  of 
rest,  whatever  path  they  may  take,  as  on  an  in- 
clined plane,  or  on  a  pendulum-rod,  etc. 


FIG.  15. 


Figure  14.  —  Accelerated  velocity 
of  falling  bodies  illustrated  by  the  flow  of 
thick  liquids.  —  Suppose  the  material  flowing 
from  the  faucet  to  be  some  thick,  tenacious 
liquid,  as  molasses  or  syrup  ;  and  though,  at  the 
faucet,  the  stream  be  an  inch  or  more  in  diame- 
ter, it  will,  if  it  fall  far,  dwindle  away  to  a  fine 
thread-like  stream  ;  but  as  no  more  of  the  liquid 
can  pass  in  any  one  part  of  the  stream  than  ano- 
ther, it  proves  that  the  liquid  moves  with  greater 
velocity  the  farther  it  falls. 


57.  Figure  15. — Reflected  motion. — It  is  a  law  of  moving 

bodies,  set  in  motion  by  a  single  force, 
to  move  forward  in  a  straight  line  until 
some  other  force  or  impediment,  act- 
ing in  a  different  direction,  changes 
their  course. 

Suppose  FL  to  be  a  floor,  made  of 
marble  or  some  other  elastic  substance, 
and  N  an  ivory  or  some  kind  of  an 
elastic  ball,  thrown  toward  the  floor 
in  the  direction  of  the  line  KX ;  it 
will  be  reflected  in  the  direction  of 
NH,  making  the  angles  KNL  and 
HNF  equal.  If  the  ball  be  thrown 
down  the  perpendicular  line,  which  is 
called  the  normal,  it  will  rebound  in 
the  same  direction.  The  angle  formed 
by  the  normal  and  KN  is  termed  the 
angle  of  incidence,  and  that  formed  by 


MOTION  AND  FORCE.  51 

the  normal  and  NH  is  the  angle  of  reflection,  and  these  angles  are 
always  equal. 

58.  Figure  16.— Resultant  motion.— This  is  produced  by  two 
or  more  forces,  termed  components,  acting  in  different  directions  on 
the  same  body  at  the  same  time. 

When  several  forces  act  on  a  body  they  may  be  arranged  in  three 
ways,  according  to  their  direction.  The  forces  may  act, 

1st.  All  in  one  direction  ; 

2d.  In  exactly  opposite  directions ;  or, 

3d.  At  some  angle. 

In  the  first  case,  the  resultant  is  the  sum  of  all  the  forces,  and  the 
direction  is  unaltered. 

In  the  second  case,  the  resultant  is  the  difference  of  the  forces,  and 
takes  the  direction  of  the  greater.  If  they  be  equal,  no  motion  is  pro- 
duced. 

In  the  third  case,  a  resultant  is  found  to  two  of  the  forces  by  the 
parallelogram  of  forces,  according  to  the  following  law,  namely:  by 
any  number  of  forces  acting  together  for  a  given  time,  a  body  is 
brought  to  the  same  place  as  if  each  of  the  forces  had  acted  on  the 
body  separately  and  successively  for  an  equal  time. 

FIG.  16. 


If  a  force  act  on  the  ball  L  in  the  direction  of  and  equal  to  the  line 
K,  it  will  pass  over  this  line  ;  but  if  there  be  simultaneously  applied 
to  the  ball  another  force,  in  the  direction  of  and  equal  to  the  line  E, 
it  will  pass  over  the  dotted  diagonal  line  V ;  and,  by  the  joint  action 
of  the  two  forces  it  will  be  moved  over  the  line  V  in  the  same  time  that 
the  first  force  would  impel  it  over  the  line  N,  and  the  second  force 
over  the  line  E,  or,  which  is  the  same,  over  their  opposite  parallel 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


FlG- 


lines.  If,  in  addition  to  these  two  forces,  a  third  force  be  simulta- 
neously applied,  equal  to  and  in  the  direction  of  the  line  A,  the  ball 
will  be  driven  over  the  heavy  line  T  ;  or,  if  it  be  impelled  by  the  result- 
ant V  and  the  force  represented  by  A  ;  or,  if  it  be  impelled  by  the 
resultant  of  A  and  N  and  the  resultant  of  N  and  E. 

These  forces  act  at  right  angles  to  each  other,  but  the  same  law  holds 
good,  whatever  be  the  angles. 

In  the  same  manner  a  i*esultant  may  be  found  for  any  number  of 
motive  forces,  by  compounding  them  two  by  two  successively. 

This  is  called  the  composition  of  forces.  By  reversing  the  operation 
a  single  force  may  be  divided  into  two  or  more  forces,  the  sum  of  which 
is  equal  to  the  one  force.  This  is  called  resolution  of  forces. 

Curvilinear  motion  will  be  illustrated  hereafter  (842). 

59.  Figure  27.—  Action  of  -wind  on  sails  of  vessels.  — 

Many  practical  examples  of  the  resolution  of  forces  might  be  given,  but 

the  sailing  of  a  boat  in  a  direction  differ- 
ent  from  the  wind  affords  a  familiar  illus- 
tration of  these  principles. 

Let  the  arrow,  crossing  the  deck  of  the 
vessel  at  right  angles  to  the  keel,  repre- 
sent the  force  and  direction  of  the  wind  ; 
then  resolve  this  force  into  two  others, 
by  forming  the  dotted  parallelogram,  and 
the  force  of  the  wind  which  falls  upon 
the  sail  NV  at  right  angles  to  its  surface 
will  be  represented  by  the  dotted  arrow  L. 
If  this  force  be  resolved  into  two  others, 
it  will  be  seen  what  amount  of  force  is 
applied  to  the  vessel  in  the  direction  of 
her  keel.  By  the  rudder,  F,  the  boat  is 
kept  in  the  proper  direction  to  receive  the 
wind  upon  the  sail  to  the  best  advantage 
with  reference  to  the  desired  course. 

To  apply  these  principles  to  the  best  ad- 
vantage, it  is  necessary  that  the  boat  be  so 
modelled  as  to  advance  as  freely  as  possi- 

ble through  the  water  in  the  direction  of  the  keel,  while  it  offers  great 

resistance  to  lateral  motion.     It  is  for  this  reason  that  sailing-vessels 

are  provided  with  keels  and  centre-boards. 


NOTION  AND  FORCE. 


53 


The  Pendulum. 

60.  Figure   18. — Compensating  pendulum.— A  pendulum 
clock,  to  run  with  accuracy,  requires  that  the  pendulum  remain  always 
the  same  length  ;  but,  as  heat  expands  and  cold  pIG 
contracts  it,  it  varies  with  the  temperature  (215). 

To  overcome  this  variation  is  the  object  of  the 
compensating  or  gridiron  pendulum. 

The  central  and  two  outer  rods,  marked  1,  are 
steel ;  the  two  intermediate  rods,  marked  2,  are 
brass.  If  the  two  outer  rods  expand,  say  an 
eighth  of  an  inch,  the  pendulum  will  be  lowered 
this  much  ;  and  as  the  central  rod  will  expand 
the  same,  it  will  be  lowered  two-eighths  of  an 
inch.  As  the  expansion  of  brass  is  twice  that  of 
steel,  the  rods,  marked  2,  will  elevate  the  pendu- 
lum, by  their  expansion,  two-eighths  of  an  inch  ; 
and  thus  the  expansion  or  contraction  of  the 
brass  just  neutralizes  that  of  the  steel  rods,  as  in- 
dicated by  the  arrows. 

Variations  in  the  vibration  of  the  pendulum 
are  effected,  at  will,  by  depressing  or  elevating  the 
pendulum-ball  by  means  of  a  screw  and  nut  at 
the  bottom,  or  by  moving  the  small  ball  on  the  central  rod. 

61.  Figure  19.— Laws  of  oscillation  of  the  pendulum.— 

Let  the  pendulum  be 
suspended  at  A;  then, 
when  it  is  in  the  position 
N  it  is  in  equilibrium, 
as  the  action  of  gravity, 
which  acts  vertically, 
is  resisted  by  the  ten- 
sion of  the  string  or 
rod.  If  the  ball  be 
drawn  aside  to  T,  and 
then  allowed  to  swing, 
gravity  acts  to  draw  it 
back  again  to  N,  where 
it  will  move  with  the 
same  velocity  as  though 
it  had  fallen  through 
the  vertical  height  from 


FIG.  19. 


54  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

T  to  the  dotted  line  MN.  In  consequence  of  its  inertia  and  acquired 
velocity,  it  will  pass  on  to  the  position  of  P.  From  T  to  N  gravity 
acts  as  an  accelerating  force,  but  from  N  to  P  as  a  retarding 
force.  Were  it  not  for  the  resistance  of  the  air,  NP  would  rigorously 

equal  NT. 

Let  AT  represent  the  tension  of  the  rod,  and  TL  the  force  of 
gravity,  then  construct  the  parallelogram  TANL,  and  TN  will  repre- 
sent the  force  with  which  the  ball  is  drawn  to  the  vertical  line  passing 
through  the  point  of  suspension,  which  continually  diminishes  as  it 
approaches  this  line,  until,  finally,  it  becomes  nothing;  when  the 
lines  TA  and  TL  will  form  a  straight  line,  and  the  forces  which  they 
represent  will  act  in  opposition. 

Laws  of  oscillation. — For  pendulums  of  unequal  lengths,  the 
times  of  oscillation  are  proportional  to  the  square  roots  of  their 
lengths. 

For  the  same  pendulum  the  time  of  oscillation  is  independent  of 
the  amplitude,  provided  the  amplitude  be  small. 

For  the  same  pendulum  at  different  places,  the  times  of  oscillation 
are  inversely  as  the  square  roots  of  the  force  of  gravity  at  those  places. 

Scientific  uses  of  the  pendulum. — The  pendulum  is  em- 
ployed to  measure  time  and  regulate  the  movements  of  clocks.  And 
as  its  oscillation  is  caused  by  the  force  of  gravity,  its  movements  are 
affected  by  whatever  affects  this  force;  hence,  it  is  a  most"  valuable 
scientific  instrument  in  the  investigation  and  application  of  principles 
relating  to  gravity,  latitude,  altitude,  shape  and  motion  of  the  earth, 
etc. 

Projectiles. 

62.  Figure  20.— Motion  of  projectiles. — Projectiles  are 
bodies  thrown  into  the  air  by  some  momentary  force,  therefore  they 
are  subject  to  two  forces :  viz.,  the  projectile  force,  which  is  moment- 
ary, and  gravity,  which  is  constant. 

If  the  body  is  projected  vertically  upward  or  vertically  downward, 
see  the  laws  of  rising  and  falling  bodies,  Fig.  13,  (55) ;  but  the  space 
traversed,  and  also  the  velocity,  are  resultants  of  the  sum  of  the  two 
forces. 

If  the  direction  of  the  projectile  is  not  perpendicular,  then  the  path 
of  the  projectile  must  be  a  curve. 

In  the  figure  suppose  the  length  of  the  narrow  parallelograms  to  be 
the  distance  the  projectile  would  travel  in  each  second,  and  their  width 
(16  feet,  Fig.  13),  the  distance  the  projectile  would  fall  in  one  second 


MOTION  AND  FORCE. 


55 


by  the  force  of  gravity.  If  now  a  cannon  ball  be  projected  from  A 
(the  gun  ranging  at  the  angle  of  45°),  it  would  be  driven,  by  the  pro- 
jectile force  alone,  the  first  second,  along  the  straight  line  toward  the 
point  L,  to  F,  but,  by  the  force  of  gravity  alone  (55),  it  would  fall  to 
the  lowest  corner  of  the  first  parallelogram  ;  hence  the  two  forces, 
acting  together,  would  drive  it  along  the  curved  line  in  the  first 


FIG.  20. 


parallelogram.  During  the  second  second  the  projectile  force,  acting 
alone,  would  drive  the  ball  from  F  to  H,  but  during  the  second  second 
gravity  would  (by  the  law  of  falling  bodies)  draw  it  down  the  width  of 
three  parallelograms,  to  the  bottom  of  the  line  3 ;  hence,  it  will  pass 
along  the  curved  line  crossing  these  three  parallelograms.  During  the 
third  second  the  projectile  force,  acting  alone,  would  drive  the  ball 
from  H  to  J,  and  gravity  would  depress  it  over  the  width  of  five  of  the 
parallelograms,  to  the  bottom  of  the  line  5 ;  hence,  it  will  pass  along 
the  curved  line  traversing  these  five  parallelograms ;  and  so  on,  until 
the  ball  falls  on  the  horizontal  line  at  T. 

It  will  reach  T  in  the  same  time  that  the  projectile  force  (unin- 
fluenced by  gravity)  would  drive  it  along  the  line  FJ  to  a  point  where 
this  line  would  intersect  a  perpendicular  erected  at  T,  which  would  be 
the  same  time  required  for  a  ball  to  fall  (by  gravity)  from  this  inter- 
section to  T. 

The  greatest  possible  horizontal  range,  with  a  given  velocity  of  pro- 
jection, is  obtained  by  placing  the  gun  at  the  angle  of  45°  with  the 


56  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

horizon ;  in  which  case  the  greatest  height  attained  is  one-fourth  of 
the  range. 

For  any  range  equally  above  or  below  45°,  the  horizontal  range  will 
be  equally  diminished ;  that  is,  the  horizontal  range  will  be  the  same 
for  40°  and  50°,  and  the  same  for  30°  and  60° ;  as  shown  by  the  other 
two  curves  in  the  diagram. 


>.  Figure  21. — Perpetual  revolution.— Suppose  the  sphere 
to  represent  the  earth,  with  a  tower  reaching  above  the  atmosphere 


FIG.  21. 


(say  fifty  miles  high),  from  which  to  project  a  cannon-ball.  A  ball 
shot  from  a  cannon  at  this  elevation,  not  being  resisted  by  the  air, 
might  be  driven  eighty  miles  or  more ;  and,  with  sufficient  projectile 
force,  a  ball  might  be  driven  completely  around  the  earth  to  the  point 
of  starting,  as  shown  in  the  diagram  ;  in  which  case  it  would  continue 
to  revolve  perpetually  around  the  earth,  same  as  the  moon. 

64.  Figure  22.— Falling  of  projectiles  thrown  from  hori- 
zontal guns. — A  ball  horizontally  projected  from  an  elevated  gun 
will  reach  the  ground  in  the  same  time  that  it  would  fall  vertically 
from  the  same  elevation,  whatever  be  the  projectile  velocity. 

Suppose  the  cannon  to  be  elevated  at  such  a  height  that  a  ball  fall- 
ing vertically  from  its  mouth  would  be  just  three  seconds  in  reaching 


MOTION  AND  FORGE. 


the  ground,  by  the  action  of  gravity,  and  the  range  of  the  gun  to  be 
exactly  horizontal. 

Suppose  a  ball  to  be  projected  from  the  gun,  and  to  reach  N  in  one 
second,  then  the  horizontal  line  Nl  will  intersect  the  vertical  line  L3 
at  the  point  to  where  an  unshot  ball  would  fall  by  gravity,  at  the  end 
of  the  first  second.  During  the  second  second,  suppose  the  projected 


FIG. 


ball  to  pass  from  N"  to  T,  then  the  horizontal  line  T2  will  intersect  the 
vertical  line  at  the  point  to  where  the  unshot  ball  would  fall  at  the 
close  of  the  second  second.  During  the  third  second,  suppose  the  pro- 
jected ball  to  pass  from  T  to  F,  then  draw  the  horizontal  line  F3 
(which  coincides  with  the  earth),  and  it  will  intersect  the  vertical  line 
at  the  point  to  where  the  unshot  ball  would  fall  at  the  close  of  the 
third  second ;  hence,  the  projected  ball  reaches  the  earth  in  the  same 
time  that  the  unshot  ball  falls  vertically  from  the  mouth  of  the  cannon 
to  the  earth. 

65.  Figure  23. — Action  and  reaction  are  equal,  or  force 
and  resistance  are  equal. — This  is  shown  by  a  series  of  ivory  or  other 
elastic  balls  suspended  by  cords.  If  the  ball  1  be  drawn  from  the  per- 
pendicular, and  then  allowed  to  fall  so  as  to  strike  the  one  next  to  it, 
the  motion  of  the  falling  ball  will  be  communicated  through  the  whole 
series  from  one  to  the  other,  without  moving  any  but  the  last.  This 


58 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


is  owing  to  the  fact  that  the  reaction  of  2  is  just  equal  to  the  action  of 
1 ;  and  that  the  reaction  of  3  is  just  equal  to  the  action  (communicated 
from  1)  of  2 ;  and  so  on,  until  the  motion  1  arrives  at  6,  which,  having 
nothing  to  act  upon,  is  itself  put  in  motion  and  thrown  off  to  L.  It 

FIG.  23. 


is,  therefore,  reaction  which  causes  all  the  intermediate  balls  to  remain 
at  rest. 

If  1  and  2  be  drawn  aside  and  allowed  to  fall  together,  then  5  and  6 
will  be  thrown  off.  In  these  experiments  elastic  balls  must  be  employed. 

The  law,  that  action  and  reaction  are  equal,  is  often  overlooked  by 
inventors  who  strive  to  produce  a  perpetual  motion. 


CHAPTEK    Y. 

MECHANICAL   POWERS. 

66.  Machine,  motor,  power,  weight,  etc. — A  machine  is 
any  contrivance  that  transmits  the  action  of  force.  A  force  employed 
to  move  a  machine  is  a  motor. 

The  moving  force  in  a  machine  is  called  the  power;  the  place  of  its 
appliance,  the  point  of  application;  the  line  in  which  this  point  tends 
to  move,  the  direction  of  the  power;  the  resistance  to  be  overcome,  the 
weight;  and  the  part  of  the  machine  immediately  applied  to  the  resist- 
ance is  the  worTting  point. 


MECHANICAL  PO  WERS.  5  9 

Forces  (or  what  is  the  same,  forces  and  resistances)  in  equilibrium 
must  he  to  each  other  inversely  as  their  velocities,  and  inversely  as  the 
spaces  which  they  describe. 

Levers. 

Figure  24.— Lever  of  the  first  class. — A  lever,  in  use,  im- 
plies an  inflexible  bar,  fulcrum,  power,  and  resistance.  Weight  is 
substituted  for  resistance  in  these  illustrations. 

In  the  first  class,  the  fulcrum  is  between  the  power  and  weight,  as 
shown  in  the  diagram,  dividing  the  lever  into  a  long  and  short  arm. 

FIG.  24. 


The  relative  length  of  these  arms  is  shown  by  figures,  and  also  by  a  cor- 
responding number  of  equal  spaces,  marked  on  the  lever ;  and  the 
relation  between  the  power  and  weight  is  indicated  by  figures — the 
ball  being  the  power. 

The  conditions  of  equilibrium  with  all  levers  are  these :  the  weight 
and  poiver  are  to  each  other  inversely  as  their  distances  from  the  ful- 
crum; or,  power  multiplied  into  its  arm  (its  distance  from  the  ful- 
crum) equals  the  weight  multiplied  into  its  arm ;  or,  as  shown  in  the 
diagram,  6  X  8  =  24  X  2 ;  or,  P.  (power)  is  to  W.  (weight)  as  short 
arm  is  to  long  arm.  Hence,  the  weight,  short  arm,  power,  or  long 
arm,  in  all  levers  may  be  found,  respectively,  by  the  following 
formulae : 

long  arm  X  P.  _  w         8  X  6  __  gl> 
short  arm  2 

long  arm  X  P.  8x6 

-  =  short  arm,  or     ^     =  2  ; 

short  arm  X  W.       ^        2  X  24 

-  =  P.,  or  — 5 —  =  6 ; 
long  arm  o 

short  arm  X  W.       ,  2  X  24 

— — —       -  =  long  arm,  or  — - —    =  8. 

These  rules  apply  to  each  of  the  other  simple  levers. 


60 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


67.  Figure  25. — Lever  of  the  second  class.— In  this  lever 
the  weight  is  between  the  fulcrum  and  power,  which  affords  greater 


FIG.  25. 


leverage  with  a  lever  of  the  same  length ;  as  it  will  be  seen  that  all  the 
ten  spaces,  instead  of  eight,  come  between  the  power  and  fulcrum, 
which  gives  the  power  a  fifth  greater  advantage  over  the  weight  than 
in  the  former  case ;  or  6  X  10  =  30  X  2  ;  thus  making  the  weight  30 
instead  of  24. 

The  dotted  lines  show  that  the  space  through  which  the  weight 
passes  is  to  the  space  through  which  the  power  passes  as  the  power  is 
to  the  weight ;  and  as  the  short  arm  is  to  the  long  arm. 


68.  Figure  26.— Lever  of  the  third  class.— In  this  lever 
the  power  comes  between  the  fulcrum  and  weight,  which  brings  the 


MECHANICAL  POWERS.  61 

weight  on  the  long  instead  of  the  short  arm,  thus  decreasing  the 
motion  of  the  power  at  the  expense  of  the  leverage ;  the  weight  having 
the  advantage  of  the  whole  length  of  the  lever,  and  the  power  only  one 
fifth  of  it;  or  30  X  2  =  6  X  10;  thus  making  the  weight  6  and  the 
power  30. 

In  this  case  the  long  arm  (that  to  which  the  power  is  usually 
applied)  becomes  the  short  arm,  and  the  short  arm  becomes  the  long 
arm,  which  must  be  kept  in  mind  when  applying  the  rules  previously 
given. 

69.  Figure  27. — Compound  levers. — A  compound  lever  con- 
sists of  two  or  more  simple  levers.  The  one  here  exhibited  consists  of 
three  simple  levers  of  the  first  class. 

FIG.  27. 


The  conditions  of  equilibrium  in  such  a  system  are,  that  the  product 
of  all  the  long  arms  multiplied  by  the  power  must  equal  the  product 
of  all  the  short  arms  multiplied  by  the  weight;  or  8x8x8X6  = 
2  X  2  X  2  X  384.  Or  the  weight  of  the  1st  lever  (beginning  next  to 
the  power)  becomes  the  power  of  the  3d  lever,  and  the  weight  of  the 
2d  lever  becomes  the  power  of  the  3d,  and  so  on.  Hence  we  have  : 

P  X  long  arm  ,  ,,7         , 

—  9  —  r-5  -  =  1st  W.,  and 

snort  arm 


lst  ^'  -  2d  W,  and 

2d  short  arm 

2d  W.  X  3d  long  arm 

-  ~~  -  s  -  =  3d  W.,  and  so  on.     Or, 
3d  short  arm 

6X8  24  X  8  96  X  8 

-  =  24,  and  —  -  —  =  96,  and  —  —  —  =  384. 

Z  A  & 

Among  the  examples  of  levers  of  the  first  class,  in  practical  use,  may 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


be  mentioned  the  crowbar,  scissors,  pincers,  snuffers,  hand-truck,  scales, 
steelyards,  etc. 

Levers  of  the  second  class  are  not  so  common.  The  crowbar,  how- 
ever, is  often  employed  as  a  lever  of  this  class,  and  nut-crackers  afford 
another  example. 

Regarding  the  practical  use  of  levers  of  the  third  class,  see  Fig.  28 
(70). 

Examples  of  compound  levers  are  found  in  various  platform  scales. 

70.  Figure  28.— Limbs  of  animals,  levers  of  the  third 
class. — Many  of  the  bones  of  animals  (including  man)  are  levers  of 
the  third  class,  moved  by  the  contraction  and  expansion  of  muscles, 
which  are  the  power ;  and  the  great  extent  and  alacrity  of  motion 
given  to  the  limbs  of  animals  are  owing  to  the  fact  that  the  muscles  are 

attached  to  the  bones  near 
thefulcrums;  as  illustrated 
by  the  human  arm  in  the 
diagram.  Let  the  bone  of 
the  forearm  be  the  lever; 
the  ball  and  forearm  itself, 
the  weight;  the  lower  end 
of  the  bone  HY,  the  ful- 
crum ;  and  the  muscle  IL, 
the  power.  If  now  the 
forearm  and  ball  be  raised, 
without  moving  the  elbow, 
it  is  evident  that  the  muscle 
must  exert  a  force  as  much  greater  than  the  weight  of  the  forearm  and 
ball,  as  the  distance  from  the  elbow  to  the  hand  is  greater  than  that 
from  the  elbow  to  the  point  of  attachment  of  the  muscle ;  and  there- 
fore the  rapidity  and  extent  of  motion  of  the  hand  will  be  correspond- 
ingly greater  than  that  of  the  contraction  of  the  muscle.  If  the  lever- 
age be  as  12  to  1,  and  the  weight  50  pounds,  the  muscle  will  exert  a 
force  of  600  pounds. 


The  Wheel  and  Axle. 

71.  Figure  29.— The  wheel  and  axle.— This  machine  is  a 
modification  of  the  lever  of  the  first  class,  and,  being  constant  in  its 
action,  it  is  sometimes  called  the  perpetual  lever.  It  consists  of  a 
cylinder,  termed  the  axle,  connected  with  a  wheel  of  much  greater 
diameter.  The  power  is  applied  to  the  circumference  of  the  wheel 
(usually  by  means  of  an  endless  rope),  and  the  weight  is  attached  to  a 


MECHANICAL  POWERS. 


63 


rope  wound  around  the  axle.     Draw  from  the  centre,  or  fulcrum,  the 
long  arm  of  the  lever,  equal  to  the  radius  of  the  wheel,  and  the  short 

FIG.  29. 


arm,  equal  to  the  radius  of  the  axle,  as  shown;  then,  as  the  conditions 

of  equilibrium,  we  shall  have,  of  course, 

W  X  radius  of  the  axle  =  P  X  radius  of  the  wheel ;  or, 

24  X  2  ::  6  X  8. 
P.  is  to  W.  as  radius  of  axle  is  to  radius  of  wheel ;  or, 

6  :  24  ::  2  :  8. 
The  power  and  weight  are  inversely  as  their  velocities. 

"Wheel  radius  X  P.  8X6 

=  W.,  or  -  - —  =  24 ; 


axle  radius 

wheel  radius  X  P. 

~WT~ 
axle  radius  X  W. 


8x6 
=  axle  radius,  or  =  2 ; 


wheel  radius 
axle  radius  X  W. 


2  X  24 

=  P.,  or — - —  =  6; 

2  X  24 

=  wheel  radius,  or  — - —  =  8. 


The  dotted  lines  may  show  that  the  entire  wheel  and  axle  are  made 
up  of  an  indefinite  number  of  simple  levers;  each,  in  its  turn,  coming 


64 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


around  to  the  horizontal  line,  and 
revolving  around  the  fulcrum. 

The  capstan,  employed  on 
ships  for  raising  the  anchor,  is  a 
modification  of  the  wheel  and 
axle. 

72.  Figure  30.— Simple 
•windlass  a  modification 
of  wheel  and  axle. — The 

axle,  in  this  case,  is  revolved  by 
means  of  a  handle,  termed  the 
winch,  or  crank,  which  is  equiv- 
alent to  a  pin  driven  into  one 
of  the  spokes  of  the  wheel.  The 
conditions  of  equilibrium  are, 
that  the 
W.  X  axle  radius  =  P.  X  length  of  the  crank. 

73.  Figure  31. — Chinese  differential  windlass  or  double 
axle. — In  this  machine  it  will  be  perceived  that  the  axle  consists  of 
two  parts  of  unequal  diameters,  and  that  the  rope  winds  around  them 

in  different  directions ; 
therefore,  every  turn  of  the 
windlass  or  handle  winds 
up  a  portion  of  the  rope 
equal  to  the  circumference 
of  the  one,  but  unwinds  a 
portion  equal  to  the  cir- 
cumference of  the  other; 
and  if  the  two  be  nearly 
equal,  the  weight  has  but  a 
slight  motion;  and,  con- 
sequently, the  power  has 
great  advantage  over  it. 
If  the  weight  rise  1  inch 
while  the  power  at  the 
handle  describes  100  inches, 
1  pound  will  balance  100 
attached  to  the  rope. 

Hence  the  conditions  of  equilibrium  are,  that  the  power  multiplied 
by  the  circumference  described  by  the  handle,  equals  the  weight  mul- 
tiplied by  the  distance  it  moves. 


FIG.  81. 


MECHANICAL  POWEE&  65 

By  this  device  space  and  time  are  conveniently  exchanged  for  power. 
Differential  pulleys,  worked  by  an  endless  chain,  are  arranged  on  the 
same  principle. 

74'  Figure  32.— Compound  wheel  and  axle. — In  the  figure 
suppose  the  small  circles  (the  axles)  to  be  cog-wheels,  working  into 
cogs  on  the  circumferences  of  the  large  wheels,  and  the  horizontal 

FIG.  32. 


diameters  to  be  three  simple  levers  of  the  same  dimensions  of  that  in 
Fig.  29  (71) ;  then  it  will  be  seen  that  the  conditions  of  equilibrium 
are  the  same  as  those  of  the  compound  lever,  Fig.  27  (69)  : 

Product  wheel  radii  X  P.  =  product  axle  radii  X  W.     Or, 
8X8X8X6=2X2X2X  384-    Or  (beginning  next  to  power), 

1st  wheel  radius  X  P.  ,  TTT 

-  =  1st  W.,  and 
1st  axle  radius 

2d  wheel  radius  X  1st  W.       ft  ,  ^r 

-  =  2d  W.,  and 
2d  axle  radius 

3d  wheel  radius  X  2  W.       0  T  nr 

-  - , .  -  =  3d  W.,  and  so  on.     Or 

3d  axle  radius 

8X6  8  X  24  8  X  96 

— - —  =  24,  and  • — - —  =  96,  and  -  — —  =  384. 

4  <i  <i 

Pulleys. 

75.  Figure  33. — Simple  fixed  or  immovable  pulley.— 

It  is  the  law  of  the  pulley  that  a  cord  or  rope,  when  stretched,  must 
have  the  same  strain  upon  it  throughout  its  length. 

The  two  vertical  cords  passing  over  the  fixed  pulley  at  the  top  of 
the  diagram,  sustain  an  equal  strain.  Allowing  nothing  for  friction 
and  rigidity  of  rope,  the  power  just  equals  the  weight;  or,  6  =  6. 

5 


66 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


FIG.  33. 


FIG.  34. 


The  horizontal  diameter  of  the  pulley  is  a  lever  of  the  first  class, 
with  its  fulcrum  at  its  centre,  the  cords  being  attached  at  the  ends  of 
its  equal  arms  T  and  L;  hence  we  have,  as  conditions  of  equilibrium, 
the  W  X  T  (its  arm)  =  P  x  L  (its  arm).  , 

The  pulley  may  be  considered  as  made  up  of  an  indefinite  number 
of  such  levers,  revolv- 
ing around  their  ful- 
crum. Therefore,  the 
only  ad  van  tages  of  this 
pulley  are  to  change 
the  direction  of  force 
and  apply  it  at  a  dis- 
tance from  its  source. 
The  object  of  the 
pulley  at  the  bottom 
is  to  again  change  the 
direction  of  the  force. 
The  cross-bar  and  ar- 
rows represent  a  whif- 
fletree  and  parts  of 
traces  of  a  harness. 


76.  Figure  34. 

— Simple  movable 

pulley.  —  In     this 

pulley  one  end  of  the 

cord  is  attached  to  a 

rigid  beam,  and   the 

other  end  is  controlled 

by  the  power ;  there- 
fore, it  is  evident  that 

the  beam  sustains  half 

the  weight   and   the 

power  the  other  half; 

for  the  pulley  acts  as  a 

lever  of  the  second 
class,  whose  arms  are  to  each  other  as  1  is  to  2  ;  the  point  T  being  the 
fulcrum ;  TL,  the  long  arm,  or  the  leverage  of  the  power ;  and  the 
distance  from  T  to  axis  of  pulley,  the  short  arm,  or  leverage  of  the 
weight.  Now,  as  the  long  arm  is  the  diameter,  and  the  short  one  the 
radius  of  the  pulley,  equilibrium  will  obtain  when  the  power  is  equal 
to  one-half  the  weight.  Or,  P  is  to  W  as  the  diameter  of  pulley  is 
to  radius  of  pulley ;  hence 


MECHANICAL  POWERS. 


67 


P  :  W  =  1  :  2— or,  in  numbers,  6   :    12  =  1  :  2;    or  P  =      ,  and 

2 

12 
TV  =  P  X  2— or,  in  numbers,  6  =  --,  and  12  =  6  X  2. 

/c 
The  power  moves  twice  as  fast  as  the  weight. 


77. 


'.  Figure  35.— Movable  and  immovable  pulley.— This 

is  a  combination  of  the  two  pulleys  previously  described ;  and,  as  the 
FIG.  35.  FIG.  36. 


fixed  pulley  affords  no  advantages  in  power,  the  conditions  of  equilib- 
rium are  the  same  as  in  the  last  case;  that  is, 

W 

P  :  W  =  1  :  2,  or  P  =  — ,  and  W  =  Px  2 ;  or,  in  numbers,  6 : 12  =  1 :  2 ; 
A 

or  6  =  i?;  and  12  =  6x2. 
A 


68 


MATTER,  FORCE,  MOTION,  AND  MECHANICS. 


78.  Figure  36.— A  system  of  pulleys  with  more  than 
one  cord. — In  this  arrangement  each  movable  pulley  holds  the  same 
relation  to  the  one  next  below  it,  that  the  lowest  one  does  to  the 
weight,  and  the  lowest  one  holds  the  same  relation  to  the  weight  that 
the  single  movable  pulley  (Fig.  35)  does  to  the  weight ;  that  is,  the 
lowest  pulley  being  sustained  by  two  cords,  if  the  weight  be  divided  by 
2  it  will  express  the  weight  held  by  the  next  pulley  above;  and  so  on. 

Hence,  equilibrium  obtains  when  the  power  equals  the  weight  divided 
by  2  as  many  times  as  there  are  movable  pulleys ;  and,  conversely,  the 
weight  equals  the  power  multiplied  by  2  as  many  times  as  there  are 
movable  pulleys.  Or, 

W 

=  2X2X2X2  and  W  =  P  X  (2  X  2  X  2  X  2). 

"K*Tn     Q7 

79.  Figure  37. — Compound  pulleys 
with  two  or  more  movable  pulleys.— 

Pulleys  of  this  kind  are  arranged  in  two  blocks? 
one  block  being  movable,  and  the*  other  im- 
movable, and  the  weight  is  divided  equally 
among  the  cords  passing  around  the  pulleys  of 
the  movable  block ;  and  as  the  power  required 
to  sustain  a  given  weight  is  diminished  one- 
half  by  a  single  movable  pulley,  it  follows  that, 
in  this  arrangement,  equilibrium  will  obtain 
when  the  power  is  equal  to  the  weight  divided 
by  twice  the  number  of  movable  pulleys. 
Hence, 

P  :  W  =  1 :  twice  the  number  of  movable  pul- 
leys; or 

P  :  W  =  1 :  number  of  cords  ;  and 
P  :  W  =  velocity  of  W :  velocity  of  P;  or  6  :  24 


P  = 

P  = 


;,  or  6  = 


num.  pulleys  X  2 

W  24 

—3-,  or  6  =  — ;  and 
num.  cords  4 


2X2 


;  and 


W  =   P  X   twice  number  pulleys;  or,  24  = 
6  X   (2  X  2) ;  and  W  =  P  X  number  cords ; 

W 

or,  24  =  6  X  4 ;  and  number  cords  =  -p- ;  or, 

-T 

Or,  as  the  weight  is  sustained  by  four  cords, 


MECHANICAL  POWERS. 


69 


and  all  being  parts  of  the  same  cord,  it  is  evident  that  each  cord  must 
bear  one-fourth  of  the  weight. 

80.  Figure  38.— Compound  pulleys  with  one  movable 
pulley. — It  is  evident,  in  this  combination,  that  as  the  weight  is  sus- 

FIG.  38.  FIG.  39. 


pended  by  three  cords,  all  being  parts  of  the  same  cord,  the  W  =  P  X 

number  of  cords;  or,  18  =  6  X  3;  and 
P  =  W  -f.  number  of  cords ;  or,  6  =  18  -5-  3  ;  and  the  number  of  cords 

=  W  -f-  P ;  or  3  =  18  -r  6  ;  and 
P  :  W  =  velocity  of  W :  velocity  of  P;  or  6  :  18  =  3  :  1. 

81.  Figure  39. — A  system  of  pulleys  with  more  than 


70 


MATTER,  FORGE,  MOTION,  AND  MECHANICS. 


FIG.  40. 


one  rope  and  three  cords  to  each  pulley. — As  the  first  pulley 
next  to  the  weight  is  sustained  by  three  cords,  all  of  which  are  parts 
of  the  same  cord,  it  is  evident  that  each  cord  will  bear  one-third  of  the 
weight ;  but  as  only  one  of  these  three  cords  attaches  to  the  second 
pulley,  of  course  each  one  of  its  cords  will  sustain  only  one-third  of 
one-third  of  the  weight ;  and  so  on.  Hence,  the 

W  =  P  multiplied  by  3  as  many  times  as  there  are  movable  pulleys ; 
and 

P  =  W  divided  by  3  as  many  times  as  there  are  movable  pulleys. 
W  =  (6  X  3  =  18),  (18  X  3  =  54),  (54  X  3  =  162),  (162  X  3)  =  486. 
P  =  (486  -5-  3  =  162),  (162  -*-  3  =  54),  (54  -*-  3  =  18),  (18  -T-  3)  =  6. 
W  :  P  =  velocity  P  :  velocity  W,  or 

6  :  486  =  1  :  81. 

Such  a  system  is  not  practical,  as  the  mo- 
tion of  the  weight  is  so  slow.  It  will  be  noticed 
that  it  would  require  81  feet  of  rope  at  the 
power  to  raise  the  weight  1  foot. 

82.  Figure  40. — A  system  of  pulleys 
with  two  ropes,  having  one  fixed  and 
two  movable  pulleys. — This  system,  by 
sailors,  is  called  the  burton,  by  means  of  which 
6  pounds  of  power  overcomes  30  of  resistance. 
For,  suppose  the  weight  to  be  30  and  the  power 
6,  the  cord  which  passes  around  T  being  at- 
tached to  the  power  and  weight,  the  power  will 
balance  6  pounds  of  the  weight,  which  leaves 
24  pounds  to  be  held  by  the  two  cords  pass- 
ing around  the  pulley  N,  half  of  which,  12 
pounds,  will  be  held  by  the  cord  passing  over 
the  fixed  pulley  L.  But  these  12  pounds 
being  again  divided  by  the  pulley  T,  will  be 
held  by  the  6  pounds  of  power ;  and,  the  power 
having  already  taken  up  6  pounds  of  the 
weight,  therefore  6  pounds  sustain  30 ;  and  we 
have  the 

P  :  W  =  6  :  30,  or  1  :  5,  or 

P  :  W  =  velocity  W  :  velocity  P. 
This  system  is  considered  quite  indispensable  on  shipboard. 


MECHANICAL  POWERS.  71 


Inclined  Plane. 

83.  Figure  41.— Inclined  plane. — An  inclined  plane  is  one 
inclined  to  a  horizontal  plane.  In  every  such  plane  three  parts  are  to 
be  considered,  height,  length,  and  base,  for  upon  the  relative  proportions 
of  these  depends  its  power.  The  advantage  gained  by  its  use  is  due  to 
the  fact  that  it  supports  a  part  of  the  iveight.  If  a  body  be  placed  on 
a  horizontal  plane  it  will  support  its  entire  weight,  but  if  the  plane  be 
gradually  elevated  at  one  end  it  will  support  less  and  less  of  it,  until 
the  plane  reaches  the  perpendicular  position,  when  it  will  cease  to  sup- 

FIG.  41. 


port  any  part  of  the  weight.  The  power  may  be  applied  in  a  direction 
parallel  to  the  length  or  parallel  to  the  base,  or  in  other  directions. 
In  any  case,  by  resolution  of  forces,  it  can  be  found  what  amount  of 
power  is  required  to  retain  a  body  upon  the  inclined  plane.  Suppose, 
in  the  figure,  the  force  to  be  applied  in  a  direction  parallel  to  the 
plane ;  draw,  from  the  centre  of  the  weight,  the  vertical  dotted  arrow, 
which  represents  the  force  of  gravity,  then  draw  the  other  dotted  arrow 
perpendicular  to  the  plane  afc  its  point  of  contact  with  the  weight ;  then 
construct  the  dotted  parallelogram,  and  the  short  side  of  the  parallelo- 
gram will  represent  the  direction  and  relative  amount  of  force  neces- 
sary to  keep  the  weight  in  equilibrium  on  the  plane. 

Suppose  the  weight  to  be  9  pounds,  length  of  plane  9  feet,  power  3 
pounds,  and  height  of  plane  3  feet,  and  the  height  and  length  divided 
off  into  equal  spaces  of  a  foot. 

In  moving  the  weight  over  one-third  of  the  plane  it  will  be  elevated 
one  foot,  as  shown  by  drawing  the  horizontal  dotted  line  from  the  W 
to  the  perpendicular ;  hence,  as  one  is  a  third  of  three,  a  force  of  one- 
third  of  the  weight  must  be  applied.  Or,  equilibrium  will  obtain  when 
the  power  is  to  the  weight  as  the  height  is  to  the  length  of  the  plane. 
Hence, 


MATTER,  FORGE,  MOTION,  AND  MECHANICS. 


P  :  W  =  height :  length,  or  3  Ibs.  :  9  Ibs.  =  3  ft.  :  9  ft. 

P  x  length 

height      ' 


W  = 


3  Ibs.  X  9         , 
or,  9  Ibs.  =  — ;  and 


W  X  neight  0  .,  9  Ibs.  X  3 

P   =    -     ,          *    ,  or,  3  Ibs.  = ;  and 

length  9 

P  X  length  3  Ibs.  X  9          , 

= Tfr— —  >  or?   3  =   — TTTv^: 5   and 


9  Ibs. 


length  = 


3  Ibs. 


8 Jf-  Figure  42. — The  screw  a  modification  of  the  in- 
clined plane. — The  screw  is  an  inclined  plane  wound  spirally  around 


FIG.  42. 


a  spindle,  and  holds  the  same  relation  to  the  ordinary  inclined  plane 
that  a  spiral  staircase  does  to  a  straight  one.  In  the  figure  a  part  of 
the  inclined  plane  L,  forming  the  screw,  is  extended  off  to  the  left  at 
the  top  of  the  spindle,  instead  of  off  to  the  right  at  the  bottom,  which 
inverts  it. 

The  thread  projects  from  the  surface  of  the  spindle,  and  fits  into 
corresponding  depressions  in  the  nut,  N.  The  point  of  the  screw 
bears  upon  an  iron  plate,  TV,  between  which  and  the  lower  beam  of  the 
frame  is  placed  the  resistance  to  be  overcome.  In  the  head  of  the 
spindle  is  a  lever  which  combines  its  power  with  that  of  the  screw. 

The  distance  between  the  threads  of  the  screw  depends  upon  the  in- 
clination of  the  inclined  plane.  Suppose  a  small  insect  to  travel  down 
the  inclined  plane  L  and  around  the  screw,  and  it  will,  at  last,  arrive 


MECHANICAL  POWERS.  73 

at  W ;  and  every  time  it  goes  around  the  screw  it  will  make  the  same 
vertical  descent  as  it  did  in  travelling  the  same  distance  on  the  straight 
portion  of  the  inclined  plane  L. 

The  resistance  bears  upon  the  inclined  face  of  the  thread,  and  the 
power  on  the  lever  parallel  to  the  base  of  the  screw.  Equilibrium, 
therefore  (without  a  lever),  will  take  place  when  the 

Power  is  to  the  weight  as  the  distance  between  the  threads  is  to  the 
circumference  of  the  screw;  and  (with  a  lever)  when  the 

Power  is  to  the  weight  as  the  distance  between  the  threads  is  to  the 
circumference  of  the  circle  described  by  the  end  of  the  lever.  Or 

P  :  \V  =  dis.  between  threads  :  sweep  of  lever. 

Supposing  the  distance  between  the  threads  is  £  an  inch  ;  length  of 
lever,  10  inches ;  and  power  6  pounds ;  and  we  have, 

P  X  sweep  of  lever  ^T       6  X  60 

W  =  -  -  ;  or,  W  = : —  =  720  ;  and 

dis.  bet.  threads  i 

p  =  Wxdis.  bet  threads  720  Xj  = 

sweep  ot  lever  60 

P  X  sweep  of  lever          6  X  60 
dis.  bet.  threads  = =^-         -;  or,  =  -J;  and 

VV  (40 

W  X  dis.  bet.  threads           720  X  4- 
sweep  of  lever  =  — p —  -  ;  or, — 

W  :  P  =  velocity  of  P  :  velocity  of  W. 
FlG  43  If  720  be  divided  by  6,  we  have 

120  Ibs.  as  the  weight  which  1 
pound  will  raise,  but  this  weight 
is  elevated  only  half  an  inch  while 
the  power  describes  120  half 
inches.  Hence,  in  this,  as  in  all 
mechanical  devices,  what  is  gained 
in  power  is  lost  in  time  and  space. 


85.  Figure  43.  —  The 
wedge  a  modification  of 
the  inclined  plane.— Instead 
of  lifting  a  load  by  moving  it  over 
an  inclined  plane,  the  same  result 
may  be  obtained  by  moving  the 
plane  under  the  load.  When  used 
in  this  way  it  is  termed  a  ivedge, 

and  usually  consists  of  two  inclined  planes  joined  base  to  base,  as 

shown  by  the  dotted  lines  in  the  figure. 


74  MATTER,  FORCE,  MOTION,  AND  MECHANICS. 

The  lack  of  the  wedge  is  that  face  to  which  the  power  is  applied ; 
the  inclined  faces,  the  sides;  and  the  distance  from  point  to  back,  its 
length. 

The  resistance  may  act  at  right  angles  to  the  sides,  as  shown  by  the 
two  transverse  arrows;  or  it  may  act  at  right  angles  to  the  length. 

In  the  first  case,  therefore,  equilibrium  obtains  when  the  power  is  to 
the  resistance  as  the  back  of  the  wedge  is  to  its  side  ;  and,  in  the  second 
case,  when  the  power  is  to  the  resistance  as  the  back  of  the  wedge  is  to 
its  length.  Or, 

P  :  AV  =  back  :  side ;  and  P  :  W  =  back  :  length. 

Hence,  in  the  first  case, 

W  X  back         ,  ^        P  X  side 

P  =  — ~T3 ;  and  W  =  — T — -; — ;  and 

side  back 

P  X  side                         W  X  back 
back  =  == ;  and  side  =  -   — p . 

And,  in  the  second  case, 

W  X  back  _        P  X  length 

P  =  — = -T- — ;  and  W  = -r-9 — ;  and 

length  back 

P  X  length                              W  X  back 
back  = ==-*  - ;  and  length  =  -    -^ . 

The  great  amount  of  friction,  and  the  method  of  applying  the  force 
in  the  use  of  the  wedge,  render  it  difficult  to  definitely  calculate  the 
power  exerted  by  it ;  but  it  will  be  readily  perceived  that  the  greater 
the  difference  between  the  length  and  back,  the  greater  will  be  the 
force  from  a  given  power.  Including  the  swinging  of  the  hammer  or 
maul,  much  time  and  space  are  exchanged  for  power. 

86.  Figure  44.— Endless  screw— and  combination  of  the 
five  mechanical  powers. — The  large  cog-wheel,  in  this  figure, 
works  into  a  screw  on  the  shaft  A,  and  this  shaft  is  worked  by  a  crank, 
which  acts  as  a  lever.  Every  time  the  crank  is  turned  the  screw  will 
turn  the  wheel  the  distance  of  the  width  of  one  tooth.  Hence,  equili- 
brium between  the  power  and  resistance  offered  by  the  teeth  of  the 
wheel,  will  obtain  when  the  power  is  to  the  resistance  as  the  distance 
between  the  threads  is  to  the  sweep  of  the  crank. 

Combination  of  the  five  mechanical  powers.— By  such  a 
combination,  could  materials  of  sufficient  strength  be  had,  there  could 
be  exerted  almost  an  unlimited  force,  but  only  through  a  correspond- 
ingly limited  space. 


MECHANICAL  POWERS. 


75 


It  is  only  by  means  of  combined  action  of  the  mechanical  powers, 
that  sufficient  force  can  be  exerted  to  haul  vessels  out  of  the  water  for 
repairs. 

FIG.  44. 


In  estimating  the  force  of  this  engine,  suppose  the  dimensions  of  its 
several  parts  to  be  as  follows  : 

Length  of  the  lever  or  crank  ....................  18  inches, 

Distance  between  threads  of  screw  ..................  1  inch, 

Diameter  of  the  toothed  wheel  ......................  4  feet, 

Diameter  of  the  axle,  L,  of  the  wheel  ................  1  foot, 

Compound  pulley,  T  ..............................  4  ropes, 

Height  of  inclined  plane,  H,  (half  its  length)  .........  2  feet, 

Power  applied  to  handle  of  the  crank  ............  10  pounds. 

The  sweep  of  the  crank  is  twice  its  length  multiplied  by  3.1416, 
which  equals  113.0976. 
Then  we  shall  have,  first, 

P  X  sweep  of  crank       10  X  113.0976 
—  ^  —  -,  —  :  —  ~j  --  ^  -  —  --  -  -  — 
dis.  bet.  threads.  1 

PXwh«n  radius       113097 


. 

i  and 


=  4533.904.  and 
axle  radius  £ 

P  X  num.  ropes  =  4523.904  X  4  =  18095.616;  and 

dg 


PX  kngth  ofpkn_e  =  18098.616XJ  = 
height  of  plane  2 

Thus  (allowing  nothing  for  the  fraction),  10  pounds  would  exert  a 
force  of  36.191  pounds;  or  a  power  of  100  pounds,  which  is  less  than 
the  power  of  a  man,  would  exert  a  force  of  361.910  pounds. 


HYDROSTATICS. 


CHAPTEK    VI. 

(CHART  NO.  2.) 

HYDROSTATICS. 

Distinguisliing  Properties  of  Solids,  Fluids,  and  Gases. 

87.  Attraction  and  repulsion. — As  previously  stated  (22),  it 
may  be  supposed  that  within  all  bodies  there  are  two  forces,  attraction 
and  repulsion.    In  rigid  bodies,  as  iron,  stone,  wood,  etc.,  the  attractive 
force  (cohesion)  preponderates,  holding  the  molecules  firmly  together, 
which  causes  the  rigidity.    In  fluid  bodies,  these  two  forces,  being  in 
equilibrium,  allow  the  molecules  perfect  freedom  to  move  in  all  direc- 
tions among  themselves,  which  causes  t\\Q  fluidity.    In  gaseous  bodies, 
the  repulsive  force  preponderates,  driving  the  particles  from  each  other, 
which  causes  the  greater  elasticity  and  compressibility  of  these  fluids. 

Definition.— Hydrodynamics  treats  of  the  peculiarities,  as  weight, 
pressure,  equilibrium,  and  motion,  of  fluid  bodies,  both  liquids  and 
gases.  It  is  subdivided  into  hydrostatics,  which  treats  of  non-elastic 
fluids  at  rest;  and  hydraulics,  which  treats  of  non-elastic  fluids  in 
motion;  and  pneumatics,  which  treats  of  the  properties  of  elastic  fluids. 

88.  Mobility  of  liquids. — Owing  to  the  equilibrium  between 
these  two  forces  (cohesion  and  repulsion),  the  particles  or  molecules  of 
liquids  are  so  free  and  mobile,  that  liquid  bodies  possess  no  definite 
form,  but  adapt  themselves  to  the  shape  of  the  vessels  that  contain 
them.     Liquids,  however,  vary  in  fluidity,  and  consequent  mobility ; 
as  between  water  or  alcohol  and  thick  viscous  bodies,  like  oils  and  tars. 

In  viscous  fluids  the  imperfect  fluidity  is  owing  to  a  greater  or  less 
preponderance  of  the  cohesive  over  the  repulsive  force,  causing  their 
molecules  to  slightly  adhere  or  stick  together.  Heat  increases  the 
repulsive  force,  and  converts  viscous  into  thin  fluids. 

With  greater  or  less  intensity  of  heat  the  repulsive  force  can  be  so 
far  increased  (or  the  cohesion  so  far  diminished)  as  to  bring  all  bodies 
not  only  to  a  fluid,  but  to  a  gaseous  condition ;  different  substances 
being  changed  from  one  to  another  of  these  states  by  adding  or 
abstracting  heat  (23) ;  as  in  the  case  of  water,  which,  when  kept  at  a 
temperature  between  32°  and  212°  F.,  is  a  liquid,  but  if  the  heat  be 
less  than  32°,  this  liquid  becomes  a  solid  (ice),  and  if  it  be  more  than 
212°,  then  it  becomes  gas,  or  an  elastic  fluid  (steam). 


HYDROSTATICS.  77 

It  is  supposed  that  the  molecules  or  ultimate  atoms  of  an  elastic 
fluid,  like  the  air,  are  as  hard  and  impenetrable  as  those  of  any  solid, 
like  iron  and  stone.  Even  though  the  air  offers  so  little  resistance  to 
other  bodies  passing  through  it,  yet  it  can  be  so  far  condensed  that, 
bulk  for  bulk,  it  will  weigh  as  much  as  the  metals. 

SO.  Compressibility  of  liquids. — Though  liquids  and  gases 
are  spoken  of  as  non-elastic  and  elastic  fluids,  yet  the  distinction  is  not 
absolute,  since  all  liquids  possess  some  elasticity.  It  has  been  shown 
that  water,  by, being  submitted  to  a  pressure  of  fifteen  thousand  pounds 
to  the  square  inch,  will  be  compressed  about  one  part  in  24;  or  about 
33  ten-rnillionths  of  its  bulk  for  each  atmosphere  of  its  pressure. 

90.  Cohesion  in  liquids. — Although   cohesion  and   repulsion 
are  spoken  of  as  being  in  equilibrium  in  liquids,  yet  there  is  a  slight 
preponderance  of  cohesion  ;  as  shown  by  their  gathering  and  adhering 
in  small  masses  or  drops  (37).     This  is  illustrated  by  the  method  of 
making  shot. 

91.  Repulsion  in  gases. — Gases  or  elastic  fluids  have  so  much 
preponderance  of  the  repulsive  force  existing  between  their  particles, 
that  they  continually  dilate  in  volume,  unless  confined  to  a  certain 
bulk  by  pressure  (23). 

Water  may  be  taken  as  the  type  or  repre- 
sentative of  fluids,  and  common  air  as  the  type 
of  gases. 

Pressure  of  Liquids. 

92.  Figure    1.  — Liquids    transmit 
pressure  equally  in  all  directions. — 

If  the  vessel  be  filled  with  water,  and  the  cork, 
N,  tightly  fitted  to  the  bottle,  and  pressed 
down  upon  the  water,  the  pressure  will  be 
transmitted  to  the  molecules  in  contact  with 
it ;  these  molecules  will  press  upon  those  next 
in  position,  and  so  on,  from  molecule  to  mole- 
cule, in  every  direction,  until  the  pressure  is 
finally  transmitted  to  every  plomt  of  the  in- 
terior surface  of  the  bottle. 

By  experiment  it  is  shown  that  the  pressure 
thus  transmitted  is  equal  to  that  applied  to 
the  cork,  surface  for  surface  ;  that  is,  the  pres- 
sure upon  each  square  inch  of  the  interior 


78 


HYDROSTATICS. 


F 


surface  of  the  vessel   is  equal   to   that   upon  a  square   inch  of  the 
cork. 

The  direction  of  the  pressure  is  at  every  point  perpendicular  to  tlie 
surface  of  the  vessel,  as  shown  by  the  arrows.  This  law  of  transmission 

of  pressure  holds  good  irrespective  of 
the  shape  and  size  of  the  vessel. 

The  whole  theory  of  Hydrostatics 
depends  upon  this  principle  of  trans- 
mission of  pressure. 

93.  Figure  2.—  Pressure  of 
liquid  not  in  proportion  to  its 
quantity,  but  to  its  height.  — 

In  this  vessel,  as  shown,  the  water  in 
the  small  division  or  pipe,  F,  stands 
on  a  level  with  that  in  the  large  divi- 
sion, T  ;  thus  showing  that  the  large 
column  of  water  presses  against  the 
small  column  with  no  greater  force 
than  the  small  column  presses  against 
the  large  one.  Hence,  columns  or 
bodies  of  liquids  of  different  magni- 
tudes, when  connected  together,  will 
be  in  equilibrium  when  they  have  the 
same  depth  or  height. 

9  Jf-  Figure  3.  —  Equilibrium  of  liquids  in  communicat- 
ing vessels.  —  A  solid  body  is  in  equilibrium  when  its  centre  of 
gravity  is  supported,  because  the  particles  of  the  body  are  held  together 
by  cohesion.  In  liquids  the  particles  do  not  cohere,  and  unless 
restrained  they  would  flow  away  and  spread  out  indefinitely.  A  liquid, 
therefore,  can  be  in  equilibrium  only  when  restrained  by  a  vessel  or  its 
equivalent. 

This  figure  represents  several  vessels,  differing  in  both  size  and 
shape,  and  all  connected  together  by  the  pipe  N.  If  either  of  these 
be  filled  with  water  to  a  given  height,  the  liquid  will  rise  to  the  same 
height  in  them  all,  as  shown.  Or,  communicating  liquids  in  different 
vessels  will  take  a  common  level,  whatever  be  the  size,  shape,  or  posi- 
tion of  the  vessels,  or  however  small  the  communicating  passage; 
because  the  pressure  of  liquids,  at  equal  depths,  is  equal  in  all  direc- 
tions. 

This  figure  is  referred  to  in  connection  with  the  explanation  of 
Figs.  9  and  10  (101  and  103). 


HTDR08TA  TICS. 
FIG.  3. 


79 


95.  Artesian  wells.— All   springs  and  fountains  illustrate  the 
law  of  equilibrium  of  liquids  in  communicating  vessels,  artesian  wells 
being  the  most  remarkable  examples.     The  water  accumulates  between 
two  impervious  strata  of  the  earth's  crust,  which  curve  up  to  the  sur- 
face of  the  ground  like  a  basin.     Suppose  a  common  bowl,  half  full  of 
dirt,  to  be  placed  in  another  a  little  larger  than  itself,  and  the  space 
between  the  two  filled  with  water.    If  now  a  tube  be  driven  down 
through  the  dirt  and  the  bottom  of  the  inner  bowl,  the  water  will  spirt 
out  of  the  tube  above  the  dirt,  as  high  as  the  level  of  the  water  between 
the  bowls.     This  is  a  miniature  Artesian  well. 

96.  Figure  4.— The  water-level. — The  surface  of  still  water 
at  any  place  corresponds  to  a  limited  horizontal  plane.     If,  however, 
the  water  extends  far,  as  in  case  of  a  lake  or  sea,  the  surface  will  be 
oval,  and  conform  to  the  shape  of  the  earth,  as  shown  in  Fig.  23  (122). 
But  practically,  for  limited  distances,  the  surface  may  be  considered  a 
horizontal  plane,  or  a  plane  at  right  angles  to  the  direction  of  gravity 
or  the  plumb-line. 

Now,  as  water  in  communicating  vessels  (94)  also  seeks  a  level,  it  is 
easy  to  construct  an  apparatus  for  finding  horizontal  lines  and  direc- 
tions with  a  small  and  portable  quantity  of  water,  as  shown  in  the 


80 


HYDROSTATICS. 


instrument  known  as  the  ivater-level ;  which  consists  of  two  upright 
tubes,  connected  at  right  angles  with  a  horizontal  tube,  as  shown  in 
the  figure.  These  tubes  are  partly  filled  with  water  or  mercury,  and 
upon  the  surface  of  the  liquid  are  placed  floats,  II,  carrying  upright 

FIG.  4. 


wires,  to  the  ends  of  which  are  attached  sights,  LL,  which  consist  of 
two  fine  hairs  or  threads,  stretched  at  right  angles  across  a  square 
frame.  The  instrument  is  mounted  on  a  stanchion,  provided  with  a 
ball  and  socket  joint.  Suppose  the  instrument  to  be  placed  in  a  hori- 
zontal position  (as  in  the  diagram),  and  the  dotted  line,  drawn  through 
the  sights  to  the  eye,  will  be  horizontal.  If  now  the  instrument  be 
lowered  at  the  left  end,  to  correspond,  for  instance,  with  the  dotted 
line  below,  the  liquid,  seeking  a  common  level,  will  rise  in  one  tube 
and  fall  in  the  other;  thus  sustaining  the  sights  on  a  horizontal  line. 
The  accuracy  will  depend  upon  the  length  of  the  horizontal  tube. 

97.  Figure  5. — The  spirit-level. — This  consists  of  a  glass  tube, 
FF,  nearly  filled  with  alcohol,  and  imbedded  in  a  piece  of  wood.  The 
bubble  of  air,  N,  will  be  equally  distant  from  either  end  when  the 
tube  is  horizontal.  If  one  end  be  raised,  as  up  to  the  dotted  line, 

FIG.  5. 


the  fluid  will  run  to  the  lower  end,  and  the  bubble  to  the  higher  end. 
This  instrument  is  chiefly  employed  by  builders,  to  level  their  work. 


HYDROSTATICS. 


81 


98.  Figure  6.— Tendency  of  liquids    to   seek   a   level 
shown  by  aqueducts. — Let  the  FIG.  6. 

dotted  line  represent  a  pipe  convey- 
ing water  over  inequalities  of  the 
earth  in  the  direction  shown  by  the 
arrows.  Whatever  be  the  head  or 
height  of  the  supply,  and  the  num- 
ber of  inequalities  over  which  the 
pipe  extends,  the  water  will  rise  and 
be  discharged  at  any  outlet  not  ver- 
tically higher  than  its  source. 

It  is  upon  this  principle  that  public  water-works  are  constructed. 

99.  Figure  7.— Intermitting  springs. — See  Siphons  (189). 

FIG.  7. 


100.  Figure  8.— Upward  pressure  of  liquids  equal  to 


downward  pressure  at  the  same 
depth. — Let  E  be  a  vessel  partly 
filled  with  water.  Take  a  tube,  F, 
with  a  movable  disk  or  false  bottom, 
N,  fitted  water-tight,  and  held  to  the 
bottom  of  the  tube  F  by  means  of 
a  cord;  then  thrust  the  tube  down 
into  the  liquid,  as  shown,  and  let 
go  the  string,  and  on  the  lower  sur- 
face of  the  false  bottom  N  the  up- 
ward pressure  will  be  equal  to  the 
downward  pressure  on  an  equal  surface 
at  the  same  depth.  This  is  proved  by 
the  fact,  that  if  water  now  be  poured 
into  the  vessel,  F,  the  false  bottom  will 
fall  off  when  the  liquid  rises  to  the 
same  level  as  that  in  the  outer  vessel. 
The  upward  pressure  of  fluids  is 
called  their  buoyant  effort. 

0 


FIG.  8. 


82  HYDROSTATICS. 

101.  Figure    9. — Downward  pressure  of  liquids  inde- 
pendent of  shape   and   capacity  of  containing  vessels.— 

The  pressure  on  the  horizontal   base  of  vessels  depends  only  on  the 
FlG  9  size   of  the   surface  pressed, 

and  its  vertical  distance  be- 
low the  upper  surface  of  the 
liquid.  Or,  the  pressure  is 
equal  to  the  weight  of  a  col- 
umn of  the  liquid,  whose  base 
is  that  of  the  vessel,  and 
whose  height  is  equal  to  the 
depth  of  the  liquid. 

Let  N  be  a  bent  tube,  filled 
up  to  the  horizontal  dotted 
line,  L,  with  mercury,  and 
let  the  three  vessels,  A,  T,  H, 
of  the  same  length  and  base, 
but  otherwise  differently 
shaped,  be  fitted  to  screw  in- 
to the  left  arm  of  the  bent 
tube ;  and  Y,  a  glass  tube 
fitted  to  the  right  arm. 

Now,  screw  the  vessel  A  to 
the  left  arm,  and  fill  it  up 
with  water,  and  the  mercury 
will  rise  in  the  right  arm  or  tube  Y,  until  the  two  liquids  are  in 
equilibrium.  Mark  the  rise  of  the  mercury  with  the  dotted  line 
Y.  Detach  the  vessel  A,  and,  in  turn,  put  on  the  vessels  T  and  H, 
and  fill  them  with  water,  and  it  will  be  found  that  the  mercury  rises 
to  the  same  height  in  every  case.  The  perpendicular  dotted  lines  in 
the  three  vessels  represent  columns  of  water  of  equal  base  and  height, 
which,  also,  indicate  equal  pressure. 

In  Fig.  3  (94),  the  vertical  dotted  lines  in  the  several  vessels  repre- 
sent, respectively,  the  columns  of  water  whose  downward  pressure  would 
equal  the  pressure  on  the  base  of  the  several  vessels;  and  the  arrows 
indicate  the  direction  of  the  pressure  in  different  parts  of  the  vessels. 

102.  Equilibrium  of  liquids  of  different  densities.— When 

liquids  of  different  densities  are  contained  in  communicating  vessels, 
they  will  be  in  equilibrium  when  the  heights  of  the  columns  are  in- 
versely as  their  densities  ;  or  (in  Fig.  9),  the  height  of  the  mercury  in 
Y  will  be  to  the  height  of  the  water  in  A  as  the  density  of  water  is 
to  the  density  of  mercury.. 


HYDROSTATICS. 


83 


FIG.  10. 


103.  Figure  10.— Pressure  of  a  liquid  is  in  proportion 
to  its  height  and  the  area  of  its  base. — Let  F  and  E  be  vessels 
of  equal  base  and  height,  but,  in 
form  and  capacity,  quite  unlike, 
each  having  for  a  base  a  disk  held 
up  by  a  cord  attached  to  one  arm 
of  a  balance,  and  sustained  with 
weights  on  the  opposite  arm. 
By  pouring  water  into  the  vessel 
E,  to  a  given  height,  and  adjust- 
ing the  weights,  the  disk  will  fall 
off  by  the  downward  pressure  of 
the  liquid,  and  the  weights  will 
indicate  the  amount  of  pressure 
on  the  base  at  the  moment  of 
separation.  If  the  string  in  F 
be  attached  to  the  arm  of  the  bal- 
ance, and  water  poured  into  the 
vessel,  up  to  the  same  height,  the 
pressure  on  the  base,  at  the  mo- 
ment of  its  separation,  will  be  the 
same,  though  much  less  liquid  is 
employed. 

The  reason  of  this  is  that  the  ^^^^^^^^^^^^^^^^^^^^ 
upward  pressure  on  the  surface  L  (shown,  by  Fig.  8,  to  be  equal  to  the 
downward  pressure),  reacts  on  the  base,  as  shown  by  the  short  arrows, 
while  in  the  other  vessel  there  is  no  upward  pressure. 

This  principle  is  further  illustrated  by  Fig.  3  (94).  If  the  second 
vessel  on  the  left  were  cut  off  from  the  other  vessels  and  filled  up  to 
the  faucet  L,  the  pressure  on  the  base  would  equal  a  column  shown 
by  the  outside  dotted  lines  up  to  the  faucet.  If  the  faucet  be  closed 
and  the  upper  part  of  the  vessel  be  filled,  the  pressure  on  the  faucet 
will  equal  the  small  dotted  column  V.  But  if  the  faucet  now  be 
opened,  the  pressure  on  the  lower  base  will  equal  the  large  dotted  col- 
umn the  full  height ;  which  is  greater  than  the  sum  of  the  two  short 
columns,  though  no  water  has  been  added. 

This  is  owing  to  the  fact  that  in  the  upper  part  of  the  vessel  some 
of  the  downward  pressure  is  supported  by  the  slanting  sides,  as  shown 
by  the  arrows;  while  in  the  lower  part,  the  pressure  on  the  base  is 
increased  by  the  upward  pressure  on  the  slanting  sides,  as  shown  by 
the  arrows. 

Again,  suppose  the  faucet  T,  in  the  central  vessel,  to  be  closed,  and 
all  the  vessels  filled,  except  H,  above  the  faucet  T  ;  then  the  pressure 


84 


HYDROSTATICS. 


FIG.  11. 


on  the  base  of  this  central  figure  will  equal  the  dotted  column  ex- 
tending the  whole  height  of  the  vessel.  If  now  the  upper  part  of  this 
vessel  be  filled,  there  will  be  a  certain  amount  of  pressure  on  the  faucet, 
but  if  the  faucet  be  opened  it  will  add  no  pressure  to  the  base  of  the 
vessel.  This  is  because  the  upward  pressure  on  the  faucet,  before  it 
was  opened,  was  just  equal  to  the  downward  pressure  from  the  water 
inH. 

104-  Figure  11. — Pressure  of  liquids  on  the  sides  of  a 
vessel. — As  liquids  transmit  pressure  in  all  directions  alike,  it  follows 
that  the  pressure  of  a  liquid  on  any  portion  of  a 
lateral  wall  is  equal  to  the  weight  of  a  column  of 
the  liquid,  which  has  for  its  base  this  portion  of 
the  wall,  and  for  its  height  the  vertical  distance 
from  its  centre  to  the  surface  of  the  liquid. 
Hence,  lateral  pressure  increases  with  the  depth 
of  the  liquid. 

To  sensibly  show  side  pressure  of  liquids,  sus- 
pend a  pail  of  water  by  a  cord,  as  in  the  figure, 
and  remove  a  portion  of  one  side  of  the  vessel, 
which  will  destroy  the  equilibrium,  and  cause  the 
pail  to  swing  to  the  side  opposite  the  opening,  as 
shown  by  the  dotted  lines.  Were  it  not  for  the 
resistance  of  the  atmosphere  on  the  stream,  the 
force  with  which  the  pail  would  be  moved  would 
equal  the  weight  of  a  column  of  the  fluid,  whose 
base  equals  the  opening ;  and  height,  the  distance 
from  the  centre  of  the  opening  to  the  surface  of 
the  liquid. 

105.  The  total  pressure  upon  the  walls 
of  a  vessel. — This  is  equal  to  the  weight  of  a 
column  of  the  liquid,  whose  base  is  equal  to  the 
area  of  the  sides,  and  whose  height  is  equal  to  one- 
half  the  depth  of  the  liquid. 

106*  The  total  pressure  on  the  bottom  and  sides  of  a 
vessel. — This  is  equal  to  the  weight  of  the  liquid  added  to  the  side 
pressure ;  and  as  the  lateral  pressure  on  one  side  of  a  cubical  vessel 
would  be  owe-half  of  the  weight  of  the  liquid,  on  the  four  sides  it  would 
be  four  times  one-half,  or  twice  the  whole  weight,  making  the  total 
pressure,  on  bottom  arid  sides,  three  times  the  weight  of  the  liquid 
contained  in  the  vessel. 


HYDROSTATICS. 


85 


107.  Figure  12.— Hydrostatic  paradox.— This  paradox  is 
another  experimental  proof  that  liquids  press  according  to  their  height 
and  not  their  quantity. 

Fill  a  glass  jar,  T,  with  water,  and  balance  it  on  a  scale-beam  with  a 


FIG.  12. 


weight,  8 ;  then  pour  out  most  of  the  water,  letting  the  balance-weight 
remain,  and  replace  the  jar,  which,  of  course,  will  not  balance  the 
weight.  If  now  there  be  introduced  into  the  jar  a  cylindrical  piece  of 
wood,  or  other  solid  substance,  a  trifle  smaller  than  the  jar,  crowding 
it  down  until  the  water  rises  to  its  former  level,  the  weight  will  again 
be  balanced;  though  the  cylinder  is  not  in  contact  with  the  jar,  and 
there  remains  but  a  small  fraction  of  the  water.  Showing  that,  if  the 
base  of  the  vessel  and  height  of  the  liquid  remain  the  same,  the  pres- 
sure  upon  the  base  will  be  the  same,  irrespective  of  the  quantity  of  ; 
liquid  employed. 

The  result  will  be  the  same,  whether  a  light  substance,  as  cork,  or  a 
heavy  material,  as  lead,  be  placed  in  the  jar  ;  the  only  condition  being, ^ 
that,  in  each  case,  the  water  shall  rise  to  the  same  height. 

The  cylinder  merely  taking  the  place  of  the  water,  it  will  have  the 
same  effect  upon  the  scale  that  its  equal  bulk  of  water  did,  which  it 
has  displaced.  Hence,  the  cylinder  may  be  any  body  which  will  dis- 
place water,  be  it  solid  or  hollo^. 


- 


80 


HYDROSTATICS. 


FIG. 


108.  Figure  13.— Practical  use  of  the  principle  that 
liquids  transmit  pressure  in  all  directions  alike. — Suppose 
T  and  L  to  be  pistons  of  equal  diameter,  fitted  water-tight  in  cylinders 
which  are  partly  submerged  in  confined  water.     If  now  there  be  placed 

upon  the  piston  heads  (T  and  L) 
equal  weights — say  5  pounds — they 
will  be  in  equilibrium.  But,  if  more 
weight  be  placed  on  one  of  the  pistons 
it  will  be  depressed  while  the  other 
will  be  raised,  and  the  equilibrium 
will  again  be  restored,  when  the 
weight  of  the  water  in  the  pistons, 
standing  between  their  respective 
levels,  equals  the  extra  weight. 

Again,  if  the  weights  are  equal  and 
the  area  of  the  pistons  unequal,  the 
equilibrium  will  be  destroyed;  for 
the  reason,  heretofore  shown  (103), 
that  the  pressure  of  liquids  is  as  the 
base  and  height.  Hence,  leaving  out 
weight  of  fluid  and  friction  of  parts, 
we  have  as  conditions  of  equilibrium 
between  force  and  resistance,  acting 
upon  pistons  through  the  interven- 
tion or  medium  of  confined  liquids 
or  fluids,  the  following  formula : 
The  force  is  to  the  resistance  as  the  area  of  the  piston  receiving  the 
force  is  to  the  area  of  the  piston  acting  upon  the  resistance. 

Or,  substituting  poiver  or  P,  for  force ;  and  weight  or  AY,  for  resist- 
ance ;  and  poiver  piston  for  the  area  of  the  piston  on  which  the  force 
acts;  and  weight  piston  for  the  area  of  the  piston  acting  upon  the 
resistance,  and  we  have : 

P  is  to  W  as  power  piston  is  to  weight  piston. 
This  is  the  principle  employed  in  the  hydrostatic  press  (109). 

109.  Figure  14.— The   hydrostatic   press.— This  press  is 
extensively  employed  for  exerting  immense  force  through  short  dis- 
tances. 

Instead  of  using  a  high  column  of  water  to  obtain  pressure  on  the 
power  piston,  for  convenience  a  lever  force-pump  is  employed. 

N  YL  is  a  strong  frame  of  wood  or  iron,  and  I  is  the  bed-plate  of  the 
press,  upon  which  is  placed  the  object  to  be  pressed,  or  the  resistance 
to  be  overcome. 


HYDROSTATICS. 


87 


The  bed-plate  is  rigidly  connected  with  the  weight  piston.  T,  by  a 
large  shaft  of  iron  ;  and  the  piston  works  in  a  heavy  iron  cylinder,  E  ; 
into  which,  and  below  the  piston,  the  water  is  forced  by  the  power 
piston  F,  with  the  lever  A  —  this  lever  and  piston  constituting  the 
force-pump.  The  water  is  drawn  by  the  pump  from  the  cistern  or 
well,  and  forced  into  the  press 
and  kept  from  returning, 
while  the  pump  is  reversed, 
by  means  of  the  ball  value,  2 
—  the  pump  value  being- 
shown  at  1. 

The  force  of  the  press  will 
depend  upon  the  power  of 
the  pump  lever,  A,  and  the 
relative  areas  of  the  two  pis- 
tons. According  to  the  for- 
mula previously  stated  (108), 
we  have  (omitting  the  advan- 
tage of  the  lever) 

W  is  to  P  as  'weight  piston 
is  to  power  piston. 

For  clearness,  substitute 
press  piston  for  weight  pis- 
ton, and  pump  piston  for 
power  piston  ;  then  we  have 

W  is  to  P  as  press  piston  is 
to  pump  piston. 

Or,  including  the  advantage  of  the  lever, 

Ttr       P  X  press  piston       , 
W  =  -  ——-      -  X  leverage. 

pump  piston 

Suppose  the  pistons  to  be  as  2  to  200  inches,  the  power  100  pounds, 
and  the  leverage  as  10  to  1  ;  and  we  shall  have,  as  the  W,  or  force  of 
the  press, 


W  = 


X  10  = 


X  10  =  100,000  pounds. 


Or,  100  pounds  on  the  lever  gives  1000  pounds  on  the  pump  piston  ; 
and  this  multiplied  by  100,  the  number  of  pump  pistons  required  to 
equal  the  press  piston,  gives  100,000  pounds. 

The  above  formula,  of  course,  will  serve  to  find  the  P,  leverage,  or 
either  piston,  when  the  other  dimensions  are  given. 


88 


HYDROSTATICS. 


The  cylinder  sjiould  be  furnished  with  a  discharge  cock  (not  shown 
in  the  diagram),  to  take  off  the  pressure  after  the  completion  of  its 
work. 

The  hydraulic  press  is  the  most  powerful  and  convenient  mechanical 
engine  in  use.  Its  power  is  limited  only  by  the  strength  of  the  ma- 
chinery and  materials  used  in  its  construction.  It  is  extensively  em- 
ployed for  pressing  cotton,  hay,  and  other  substances  into  bales,  raising 
ships  for  repairs,  testing  the  strength  of  cables,  pipes,  steam-boilers,  etc. 

110.  Figure   15. — Bursting  a   cask   with,   hydrostatic 
pressure. — To  further  illustrate  that  the  pressure  of  a  liquid  does 
not  depend  upon  its  bulk,  but  upon  its  height  and  base,  take  a  cask 
holding  50  or  60  gallons,  fill  it  with  water,  and  into  its  head 'insert  a 
tube,  T,  40  or  50  feet  long.     If  this  tube  be  filled  with  water,  which  in 
quantity  need  not  be  more  than  a  pint,  it  will  burst  the  cask,  exert- 
ing as  much  pressure  on  its  interior  surface  as  if  the  sides  of  the 
cask  were  extended  up  in  the  direction  of  the  arrows  to  the  full  height 
of  the  tube,  and  then  filled  with  water. 

This  serves  to  show  also  the  upward  pressure  of  liquids ;  for  if  a 
stop-cock  be  inserted  in  the  upper  head  of  the  cask,  and  opened  when 
the  tube  is  kept  full,  the  water  will  spirt  up  nearly  as  high  as  the  tube. 

111.  Figure  16.— Hydrostatic  bellows.— This  is  still  ano- 
ther instrument  to  illustrate  the  great  pressure  of  a  small  vertical 
column  of  liquid. 

The  bellows  is  made  of  leather  nailed  to  two  circular  disks  of 
wood,  having  a  vertical  pipe,  P,  opening  into  the  interior.  If  it  be 
filled  with  water  to  the  top  of  the  tube,  the  upward  and  downward 
pressure  on  the  disks  will  equal  the  weight  of  a  column  of  water  whose 
base  is  equal  to  the  face  of  the  disks,  and  whose  height  equals  that  of 
the  tube. 

A  pint  of  water,  in  this  instrument,  may  be  made  to  elevate  thou- 
sands of  pounds.  The  arrows,  if  extended  as  high  as  the  tube,  would 
show  the  size  of  the  column  of  water,  whose  weight  would  equal  the 
pressure  on  the  disks ;  or,  the  pressure  on  one  of  these  disks  will  be 
to  the  weight  of  the  water  in  the  tube  as  the  area  of  one  of  the  disks 
is  to  the  area  of  the  tube. 

Striking  effects  of  the  pressure  of  water,  by  its  own  weight,  are  ex- 
hibited in  the  ocean.  A  strong  square  glass  bottle,  empty  and  firmly 
corked,  will  have  its  sides  crushed  in  by  being  sunk  in  water,  at  a 
depth  less  than  sixty  feet.  Divers  cannot  descend  far  below  the  sur- 
face, and  even  fish  cannot  descend  beyond  a  given  limit,  in  conse- 
quence of  the  increased  pressure  of  the  water. 


HYDROSTATICS. 


89 


FIG.  15. 


FIG.  16. 


90 


HYDROSTATICS. 


112.  Figure  17.— 
Hydrostatic  pressure 
in  mountains. — Suppose 
L  to  represent  a  small  pool 
of  water,  of  only  a  few 
yards  in  extent,  but  several 
hundred  feet  below  the  sur- 
face water,  at  W ;  and  F  a 
small  passage  leading  from 
W  to  L ;  then  the  pressure 
on  the  pool  of  water  would 
be  equal  to  the  weight  of  a 
column  of  water  whose  base 
is  equal  to  the  surface  of  the 
pool,  and  whose  height  is 
equal  to  the  distance  from 
L  to  AV.  Of  course,  it  mat- 
ters not  how  shallow  be  the 
water  in  the  pool,  or  how 
small  the  stream,  F,  that 
supplies  it.  It  is  not  improbable  that  mountains  have  thus  been  rup- 
tured. 

113.  Figure  18.— Submerged  bod- 
ies not  pressed  in  all  directions 
equally. — Suppose  a  cube  to  be  immersed 
in  water,  as  shown.  The  opposite  lateral 
faces  will  be  equally  pressed,  and  in  opposite 
directions,  as  indicated  by  the  arrows. 

The  lower  side  will  be  pressed  upward  by 
a  force  equal  to  the  weight  of  a  column  of 
the  liquid  whose  base  is  that  of  the  cube, 
and  whose  height  is  the  distance  from  its 
lower  face  to  the  surface  of  the  fluid.  The 
pressure  on  the  upper  face  will  be  downward, 
and  equal  to  the  weight  of  a  column  of  the 
liquid  laterally  as  large  as  the  cube,  and 
whose  height  equals  the  distance  from  the 
top  side  of  the  cube  to  the  surface  of  the 
liquid ;  and  the  resultant  of  these  two  press- 
ures is  an  upward  force,  equal  to  the  weight 
of  a  volume  of  the  liquid  equivalent  to  that 
of  the  cube. 

This  upward  pressure  is  the  buoyant  effort 


FIG.  18. 


HYDROSTA  TICS. 


91 


of  the  fluid.  Hence,  a  submerged  body  displaces  a  quantity  of  fluid 
equal  to  its  own  bulk,  and  loses  a  portion  of  its  weight  equal  to  that 
of  the  fluid  displaced  by  it. 

Specific  Gravity. 

11£.  Specific  gravity. — By  specific  gravity  of  a  body  is  meant 
its  weight,  compared  with  that  of  another  hody  of  the  same  magni- 
tude, assumed  as  a  standard,  or  its  relative  weight.  Distilled  water,  at 
60°  F.,  is  taken  as  the  standard  for  solids  and  liquids.  If  a  cubic 
inch  of  gold,  for  example,  weighs  19  ounces,  and  a  cubic  inch  of  water 
1  ounce,  it  shows  that  the  relative  weight  of  water  and  gold  is  as  1 
to  19 ;  or,  that  the  specific  gravity  of  gold  is  19,  being  19  times 
heavier  than  water. 

115.  Figure  19. — Method  of  finding  specific  gravity  of 
solids. — The  body  (if  not  lighter  than  water)  is  suspended  by  a  hair 

FIG.  19. 


or  a  fibre  of  silk  to  the  scale-beam,  and  weighed  out  of  water,  in  the 
air,  as  shown  in  the  figure,  where  its  weight  is  9  pounds.  It  is  then 
weighed  in  water  (by  lengthening  the  string),  where  its  weight 
is  6  pounds,  showing  that  the  loss  is  3  pounds.  The  bulk  of  the 
water  displaced  (shown  between  W  and  3)  is  equal  to  the  bulk  of 
the  body,  and  weighs  3  pounds  (shown  by  the  loss  of  weight  in  the 


92 


HYDRO ST A  TICS. 


object) ;  therefore,  the  body  is  as  many  times  heavier  than  water  as  3 
is  contained  in  9. 

RULE. — Divide  the  weight  of  the  body  out  of  water  by  the  loss  of 
weight  in  water.  Or, 

weight  out  of  water  9  _ 

P*  gr*  ""  loss  of  weight  in  water'  °r'  3  -= 

For  solids  lighter  than  water. — When  the  body  is  lighter  than  water, 
it  must  be  attached  to  some  solid  (whose  weight  in  air  and  water  is 
known)  sufficiently  dense  to  sink  it  in  water.  The  compound  mass 
being  weighed  in  air  and  water,  and  the  loss  determined,  subtract  the 
loss  of  the  heavy  body  from  the  loss  of  the  compound  body,  and  di- 
vide the  weight  of  the  light  body  in  air  by  the  difference  of  these 
losses. 

EXAMPLE. — A  substance  weighed  in  air  200  grains.  Attached  to  a 
piece  of  copper,  it  weighed  in  air  2247  grs.,  in  water  1620  grs.,  suffering 
a  loss  of  627  grs.  The  copper  itself,  when  weighed  in  water,  lost  230 
grs.  Difference  of  losses  is  627  less  230  =  397  ;  then  we  have, 


Sp.  gr.  = 


200 


627  -  230 


=  .504. 


For  liquids. — Select  some  heavy  body,  as  a  cubic  inch  of  lead,  and 
weigh  it  in  air,  then  in  water,  and  finally  in  the  liquid  in  question. 
Subtract  the  second  and  third  weights  from  the  first  separately ;  and 
FIG.  20.  ^ne  results  obtained  will  be  respectively  the  weights 

of  a  volume  of  water  and  of  the  liquid,  equal  to  that 
of  the  cube.  Divide  the  latter  by  the  former,  and 
the  quotient  will  be  the  specific  gravity  required. 

Suppose  the  cube,  weighed  in  water,  loses  253  grs., 
and  in  alcohol  only  204  grs. ;  then  the  weight  of  al- 
cohol and  water  will  be  to  each  other  as  these  losses, 
or  as  253  to  204 ;  or,  the  sp.  gr.  of  alcohol  is  204 
divided  by  253  =  .809  + . 


116.  Figure  20.  — Specific  gravity  of 
liquids,  continued — Hydrometer. — The  hy- 
drometer is  an  instrument  by  which  the  specific 
gravities  of  liquids  are  ascertained  from  the  depth 
to  which  the  instrument  sinks  below  the  surface. 
It  consists  of  a  light  glass  tube  with  a  hollow  ball 
or  float,  L,  attached  to  one  end,  and  on  the  ball,  op- 
posite the  tube,  is  fastened  a  small  piece  of  metal, 
T,  to  keep  the  instrument  upright  in  the  liquid. 


HYDROSTATICS. 


93 


Within  the  tube  is  a  printed  graduated  scale.  The  scale  is  made 
thus:  the  instrument  is  adjusted  to  sink  in  water,  up  to  the  point, 
say,  midway  from  the  ball  to  the  top  of  the  tube.  This  point  is  marked 
1  on  the  tube  (water  being  the  standard),  and  above  and  below  this 
mark  others  are  made,  which  indicate,  in  weight,  grains.  For  conve- 
nience, therefore,  the  1  is  marked  1,000,  standing  for  1,000  grains,  as 
the  weight  of  water.  Above  this  the  numbers  decrease,  and  below, 
increase. 

Now  if  the  instrument  be  placed  in  alcohol,  which  is  lighter  than 
water,  it  will  sink  down  to  .809,  which  indicates  the  specific  gravity. 

In  a  liquid  heavier  than  water,  it  would  stand  higher  than  1,  or 
1.000. 

The  specific  gravity  of  liquids  of  commerce,  as  alcohol,  acids,  solu- 
tions, milk,  etc.,  being  well  established,  this  instrument  becomes  a  con- 
venient means  to  determine  whether  or  not  they  have  been  diluted 
with  water,  or,  in  many  cases,  otherwise  adulterated. 


FIG.  21. 


117.  Figure  21.— Liquids  of  unequal  density  seek  dif- 
ferent ,  levels  in  the  same  vessel.— If  two 
or  more  liquids,  which  do  not  chemically  or  me- 
chanically unite,  are  poured  into  the  same  vessel, 
they  will  adjust  themselves  one  above  another,  in 
the  order  of  their  respective  specific  gravities ;  the 
heaviest  falling  to  the  bottom,  and  the  lightest 
rising  to  the  top. 

The  figure  represents  a  glass  jar  containing 
three  liquids  of  unequal  specific  gravities ;  viz. : 
mercury,  water,  and  oil. 

It  is  in  accordance  with  this  principle  that 
cream  rises  on  milk,  oil  on  water,  etc. 

118.  Figure  22. — Principles  of  nota- 
tion.— When  a  body  is  plunged  into  a  liquid,  it 
is  urged  downward  by  its  weight,  and  upward  by 
the  buoyant  effort  of  the  liquid.  Three  cases 
may  arise,  depending  on  the  relative  intensities 
of  these  forces  :  1st,  when  the  density  of  the  body 
is  greater  than  that  of  the  liquid ;  2d,  when  the 
density  of  the  body  is  less  than  that  of  the  liquid ; 
and,  3d,  when  the  density  of  the  body  and  liquid  are  equal. 

When  a  floating  body  comes  to  rest,  the  plane  of  the  upper  surface 
of  the  liquid  is  called  the  plane  of  flotation. 

When  a  body  is  so  shaped  as  to  displace  more  than  its  bulk  of  liquid, 


PNEUMATICS. 


FIG.  22. 


as  in  the  case  of  a  hollow  dish,  it  may  float,  though  the  density  of  the 
material  be  many  times  greater  than  that  of 
the  liquid.  This  is  the  case  with  iron  ships. 
The  "  Great  Eastern,"  though  it  is  the  heaviest 
movable  object  in  the  world,  and  made  of  iron, 
is  as  buoyant  and  light  on  the  water  as  a  bam- 
boo stick. 

The  figure  is  a  toy  to  illustrate  the  prin- 
ciples of  flotation.  The  hollow  ball,  when 
partly  filled  with  water,  and  fastened  to  the 
metallic  fish,  is  adjusted  to  float  just  below  or 
at  the  surface  of  the  water ;  that  is,  so  that 
the  specific  gravity  of  the  apparatus  will  just 
equal  that  of  the  water ;  and  the  jar  covered 
air-tight,  with  a  rubber  or  some  other  elastic 
cap. 

If  now  the  cap  be  pressed  down  with  the 
fingers,  the  air  above  the  liquid  will  be  com- 
pressed, and  the  pressure  communicated  to 
the  water  (as  indicated  by  the  arrows),  thence 
to  the  air  in  the  ball,  which  will  be  compressed 
by  the  water  being  driven  into  the  small  open- 
ing on  the  lower  side  of  the  ball,  which,  of  course,  increases  the  weight, 
or  rather  diminishes  the  buoyant  effort  of  the  toy,  and  thus  causes  it 
to  sink  to  the  bottom.  If  the  pressure  on  the  cap  be  removed,  the 
elasticity  of  the  air  in  the  ball  will  drive  the  water  out  of  the  opening, 
and  so  increase  the  buoyancy,  when  the  toy  will  again  rise  to  the  sur- 
face. 

This  is  similar  to  the  process  in  fish,  their  "  air-bladder  "  taking  the 
place  of  the  ball. 


CHAPTER    YII 
(CHART  NO.  2.) 
PNEUMATICS. 

11 9.  Definitions. — Pneumatics  treats  of  the  properties  of  elastic 
fluids;  which  may  be  divided  into  two  classes,  gases  and  vapors. 

In  gases,  the  molecular  force  of  repulsion  (22,  23,  88,  91)  prevails 
over  the  force  of  attraction ;  and  in  permanent  gases  this  force  has 
never  been  overcome. 


PNEUMATICS.  95 

Vapors  differ  from  gases  chiefly  in  that  they  are  produced  by  the 
action  of  heat  upon  liquids  (as  steam  from  water),  and  by  their  return- 
ing again  to  the  liquid  state  by  loss  of  heat. 

Tension  is  an  expression  for  the  tendency  of  a  gas  to  expand. 

120.  Gases,  simple  or  compound.— Of  the  thirty-four  known 
gases,  four   only  are   simple   or   elementary,  viz.:   oxygen,   nitrogen, 
hydrogen,  and  chlorine.     The  first  three  of  these,  together  with  the 
compound  gases,  oxyd  of  carbon  and  bynoxide  of  nitrogen,  are  the  only 
aeriform  bodies  which  have  not,  by  cold  and  pressure,  been  reduced  to 
the  liquid  or  solid  state.     Hence  they  are  termed  permanent  or  inco- 
erciUe  gases. 

Expansion  is  the  most  characteristic  property  of  gases ;  and,  for  all 
that  appears,  this  molecular  force  would  dilate  them  indefinitely  through 
all  space,  were  there  no  counteracting  causes. 

121.  Mechanical  condition  of  gases. — Perfect  freedom  of 
motion  among  their  particles,  and  being  also  elastic,  ponderable,  and 
impenetrable,  it  follows  that  all  the  characteristic  properties  of  liquids 
apply  also  to  gases.     Hence,  they  transmit  pressure  in  all  directions 
alike,  have  buoyancy,  inertia,  specific  gravity,  etc. 

Atmospheric  air  is  the  type  of  permanent  gases,  and  is  employed  as 
the  standard  of  weight  for  gases. 


Atmospheric  Air. 

.  Figure  23. — The  atmospheric  air  an  aerial  ocean 
enveloping  the  earth. — This  diagram  shows  the  globular  form  of 
the  earth,  its  uneven  surface,  and  the  relation  of  its  surface  to  the 
water  and  atmosphere.  The  land-surface  of  the  earth,  instead  of  being 
even,  as  might  be  shown  by  the  true  interior  circle  of  the  figure,  is 
rough  and  jagged,  as  illustrated  by  the  line  drawn  above  and  below  the 
water.  The  inequalities  shown  by  the  figure,  however,  are  immensely 
exaggerated.  About  three-quarters  of  the  earth's  surface  is  covered  with 
water,  filling  up  its  inequalities.  Yet  there  are  mountains  extending 
five  miles  above  the  water ;  and  when  the  earth  is  viewed  within  the 
narrow  limits  of  vision  it  seems  to  be  exceedingly  rough  and  uneven. 
Yet,  relatively  to  its  size,  it  is  smoother  than  an  orange ;  and,  con- 
sidering its  elastic  or  atmospheric  envelope,  its  surface  is  relatively 
softer  than  velvet,  and  smoother  than  polished  steel. 

If  vision  could  extend  from  the  eye,  in  the  diagram,  to  F,  it  would 
take  in  mountain-tops  thousands  of  miles  apart ;  but  when  it  is  con- 
sidered that  the  distance  between  the  two  radii,  drawn  from  E  to  N,  is 


96 


PNEUMATICS. 


over  a  thousand  miles,  it  is  plain  that  within  the  reach  of  vision  (say, 
twenty-five  to  thirty  miles),  the  surface  of  the  water  would  appear  to 
be  a  horizontal  plane,  instead  of  a  portion  of  a  sphere. 

The  outer  circle  in  the  figure  represents  the  limit  of  the  atmosphere, 
but  its  distance  from  the  land  and  water  surface  of  the  earth,  relatively 
to  the  size  of  the  drawing,  is  far  too  great. 

FIG.  23. 


123.  Height  of  the  atmosphere  .—The  atmosphere  does  not 
extend  more  than  fifty  miles  from  the  land  and  water  ;  therefore,  rela- 
tively, it  is  a  mere  film  on  the  face  of  the  earth.  On  a  twelve-inch 
globe  it  would  be  less  than  an  eighth  of  an  inch  in  depth.  It  is  the 
most  dense  at  the  surface  of  the  earth,  and  becomes  exceedingly  rare- 
fied at  its  greatest  altitude.  Yet  it  is  held  to  the  earth  and  kept  in 
equilibrium,  against  its  elastic  or  repulsive  force,  by  gravity.  And 
though  the  earth  is  moving  around  the  sun  at  the  rate  of  68,000  miles 
an  hour,  the  atmosphere  is  not  disturbed,  showing  that  the  planets 
meet  with  no  resistance  in  space. 


'  Composition  of  the  atmosphere.  —  Air  is  chiefly  com- 
posed of  the  two  incoercible  gases,  nitrogen  and  oxygen,  in  the  propor- 
tion of  79  parts  of  the  former  and  21  parts  of  the  latter. 

Though  oxygen,  when  thus  mixed  with  nitrogen,  supports  animal 


PNEUMATICS. 


97 


life  and  combustion,  it  would,  of  itself  alone,  so  FIG.  24. 

intensify  combustion  and  the  animal  functions, 
as  to  destroy  life,  and  burn  up  the  world ;  while 
nitrogen,  of  itself,  would  not  support  life  and 
combustion  at  all.  Hence,  the  purpose  of  nitro- 
gen seems  to  be  to  dilute  the  oxygen. 

Respiration  of  animals  and  combustion  con- 
sume oxygen  and  supply  carbonic  acid,  while  the 
growth  of  vegetables  consumes  carbonic  acid  and 
throws  off  oxygen.  The  air,  therefore,  contains, 
besides  oxygen  and  nitrogen,  small  quantities  of 
carbonic  acid  (from  1  to  2  parts  in  2,000),  and 
also  variable  quantities  of  vapor  of  water  (with- 
out which  we  could  not  breathe  it),  and  traces  of 
ammonia. 

125.  Figure    24.— Impenetrability  of 
gases. — Let  W  be  a  glass  jar  partly  filled  with 
water,  and  A,  a  glass  cylinder  with  a  faucet  at 
the  top,  as  shown.     If  the  faucet  be  closed,  and 
the  cylinder  pushed  to  the  bottom  of  the  contain- 
ing vessel,  the  water  will  rise  higher  outside  than  inside  the  cylinder, 
owing  to  the  impenetrability  of  the  air  within ;  and . 

equilibrium  will  be  established  between  the  elasticity  FIG.  25. 

of  the  air  and  the  upward  pressure  of  the  water.     If 

the  faucet  now  be  opened,  the  air  will  escape,  and 

the  water  will  rise  within  the  cylinder  until  it  finds 

a  common  level  in  the  containing  vessel ;  showing 

that  it  was  the  resistance  of  the  air  that  kept  the 

water  from  rising  before  the  faucet  was  opened. 

1 26.  Figure  25. — Pressure  or  weight  of 
the  atmosphere. — Fit  into  a  strong  glass  cylin- 
der a  piston,  provided  with  a  valve  opening  outward, 
and  crowd  it  down  to  the  bottom  of  the  cylinder. 
As  it  is  pushed  down,  the  air  in  the  cylinder  will 
open  and  pass  through  the  valve,  as  shown  by  the 
arrow.     When  the  piston  has  reached  the  bottom  of 
the  cylinder,  the  valve  will  close  by  its  own  weight. 
If  now  the  piston  be  drawn  up,  no  air  can  find  its 
way  into  the  cylinder  below  it,  consequently  there 
can  be  no  upward  pressure  of  air  acting  upon  it; 
and  as  the  pressure  on  the  upper  surface  can  be  ac- 


98 


PNEUMATICS. 


counted  for  only  by  the  downward  pressure  of  the  air  (shown  by  the 
small  arrows),  it  is  taken  for  its  weight ;  which,  it  has  been  found  by 
experiment,  is  15  pounds  to  the  square  inch,  at  the  level  of  the  sea. 
Hence,  as  fluids  press  in  all  directions  alike,  the  pressure  of  the  atmo- 
sphere upon  every  object  in  every  direction  is  equal  to  15  pounds  per 
square  inch. 

The  surface  of  the  human  body,  being  about  2,000  square  inches,  is 
submitted  to  an  atmospheric  pressure  of  30,000  pounds,  or  about  15 
tons ;  which,  of  course,  could  not  be  sustained,  only  that  it  presses  in- 
ternally, externally,  upward,  downward,  and  laterally  alike. 

Every  physical  pore  of  all  organic  structures,  animal  and  vegetable, 
all  the  pores  of  the  ground,  every  nook,  hole,  and  crevice  in  the  rocks 
and  hills,  are  thus  filled  with  and  pressed  by  the  atmosphere. 


FIG. 


127.  Figure  26. — Compression  and  expansion  of  the 
itmosphere. — Provide  a  strong  cylinder,  N,  having  a  faucet,  T,  at 
the  bottom,  and  a  tightly-fitted  piston ;  enter 
the  piston  at  the  top,  and,  by  pressing  it  down 
with  sufficient  force,  the  air  within  can  be  com- 
pressed into  a  hundred  times  less  than  its  usual 
bulk.  Air  is  found  to  be  compressible  in  pro- 
portion to  the  force  employed ;  that  is,  the  bulk 
of  a  given  weight  of  air  is  reduced  to  one-half 
by  doubling  the  original  pressure,  to  one-third 
by  trebling  it,  and  so  on. 

Expansion. — Open  the  faucet,  T,  and  drive 
out  most  of  the  air  by  crowding  the  piston 
down  to  within  an  inch  or  so  of  the  bottom  of 
the  cylinder,  then  close  the  faucet  and  draw  the 
piston  up,  and  the  small  quantity  of  air  will 
expand  and  fill  the  entire  cylinder. 

It  is  found  that  the  air,  when  its  usual  pres- 
sure is  diminished,  expands  in  the  same  ratio 
that  it  condenses ;  that  is,  if  half  the  pressure 
is  removed  from  a  given  weight  of  air,  it  will 
occupy  twice  the  space  it  did  before ;  if  sub- 
jected to  one-third  the  first  pressure,  three 
times  the  space ;  and  so  on.  Air  has  been  condensed  27  times,  and 
expanded  112  times.  See  147. 


128.  Figure   27.— Air-pump,  receiver,   and  vacuum. — 

The  air-pump  is  an  apparatus  to  draw  the  air  out  of  vessels.     The 


PNEUMATICS.  99 

vessel  so  exhausted  is  called  a  receiver;  and  the  space  within,  thus 
deprived  of  air,  is  termed  a  vacuum. 

In  the  figure,  the  inverted  glass  vessel,  W,  is  the  receiver,  fitting  air- 
tight on  a  smooth  surface,  called  the  plate.  In  this  plate  is  an  aper- 
ture into  which  is  fastened  a  pipe,  I,  that  communicates  with  the 
pump  N". 

OPERATION. — As  the  piston  is  drawn  up  to  L,  the  upper  valve  (sit- 
uated in  the  centre  of  the  piston)  closes  by  its  own  weight  and  down- 
ward pressure  of  the  air,  and  the  downward  pressure  on  the  lower 
valve,  T,  being  thus  removed,  the  air  in  the  receiver  will,  by  expansion, 

FIG.  27. 


open  and  pass  through  it;  as  shown  by  the  arrow  passing  into  the 
pipe  and  through  the  valve.  Now,  if  the  piston  be  pushed  down,  the 
lower  valve  will  close,  and  the  dilated  air  in  the  pump  will  be  com- 
pressed and  pass  through  the  upper  valve.  By  again  raising  the  piston, 
the  air  in  the  receiver  will  be  further  expanded ;  and  so  on  ;  until  the 
air  is  so  intensely  rarefied,  that  it  will  no  longer  open  the  valves,  when 
the  process  must  cease.  The  pump  does  not,  therefore,  produce  a  per- 
fect vacuum. 

129.  Various  phenomena  in  vacuo. — The  unlighted  lamp 
under  the  receiver,  indicates  that,  without  air,  there  can  be  no  combus- 


100 


PNEUMATICS. 


FIG.  28.  tion  ;  the  dead  bird,  that  without  air 

there  can  be  no  flight  or  animal  life  ; 
the  coin  and  leather,  that  in  a 
vacuum,  light  and  heavy  bodies  will 
fall,  ly  force  of  gravity,  with  equal 
rapidity  ;  the  inverted  flask  of  water, 
that  without  the  downward  pressure 
of  the  air  on  liquids,  the  "suction-" 
pump  will  not  operate. 

ISO.  Figure  28.— Pressure 
of  air  equal  in  all  directions 
shown  by  hollow  hemispheres. 

— This  apparatus  consists  of  two 
brass  hemispheres,  fitted  to  each 
other  air-tight,  and  provided  with 
handles.  In  the  shank  of  the  handle 
of  one  of  them  is  a  faucet,  and  a  pipe, 
H,  to  connect  them  with  the  air- 
pump.  Having  placed  them  together 
and  attached  them  to  the  pump,  ex- 
haust the  air.  It  will  then  require,  to  separate  them,  a  force  equal  to 
FIG.  29.  15  pounds  to  the  square  inch  of  the 

surface  between  them.  This  is  the 
case,  whichever  side  up  they  are  placed, 
which  proves  that  the  pressure  of  the 
atmosphere,  which  holds  them  together, 
is  the  same  in  all  directions.  These 
hemispheres  have  been  made  so  large  • 
as  to  require  fifteen  horses  on  each  side 
to  draw  them  asunder ;  yet  by  opening 
the  faucet  they  would  fall  apart. 

131.  Figure  29.— Expansion 
fountain. — This  consists  of  two  glass 
globes,  the  upper  one  being  open  at  the 
top,  and  furnished  with  a  faucet  and 
tube,  the  tube  being  open  at  the  bottom, 
and  reaching  nearly  to  the  bottom  of 
the  lower  globe.  The  lower  one  being 
nearly  filled  with  colored  liquid,  the 
upper  one,  with  the  pipe,  is  fastened 
to  it,  and  (with  faucet  open)  placed 


PNEUMATICS.  V  /^  ^  ,  ' 

under  the  receiver  of  the  air-pump,  as  seen  in  the          FIG.  80. 
figure. 

If,  now,  the  air  be  exhausted  from  the  receiver,  the 
air  above  the  water  in  the  lower  globe  will  expand 
and  press  upon  the  water  (as  shown  by  the  arrows), 
and  drive  it  up  through  the  pipe  into  the  upper  globe. 
By  allowing  the  receiver  to  be  filled  with  air,  the  water 
will,  by  its  gravity,  return  again  to  the  lower  globe. 
This  process  can  be  repeated  any  number  of  times. 


132.  Figure  30.— Atmospheric  pressure.— 
It  varies  with  variations  of  altitude.— Suppose 
the  figure  to  represent  a  tube  not  less  than  thirty-five 
feet  long,  having  a  tightly  fitted  piston,  with  a  rod  or 
handle  as  long  as  the  tube.  Set  the  lower  end  of  the 
tube  into  water,  so  that  the  water  will  be  in  contact 
with  the  lower  side  of  the  piston.  If,  now,  the  piston 
be  drawn  up  by  the  handle,  it  will  remove  the  down- 
ward pressure  on  the  water  within  the  tube,  and  the 
downward  pressure  on  the  water  outside  the  tube  will 
force  it  up  under  the  piston,  as  shown  in  the  figure, 
until  the  weight  of  the  column  of  water  will  equal 
the  atmospheric  pressure,  which,  as  heretofore  shown, 
is  15  pounds  to  the  square  inch. 

The  height  to  which  the  water  will  thus  rise  is  about 
33  feet  9  inches.  Hence,  the  pressure  or  weight  of  a 
column  of  water  33  ft.  9  in.  high  is  equal  to  the  press- 
ure or  weight  of  an  equal  column  of  air  as  high  as 
the  atmosphere  extends,  be  the  height  more  or  less. 

If  the  piston  be  drawn  up  higher  than  33  ft.  9  in. 
the  water  will  cease  to  follow  it,  and  the  space  between 
it  and  the  water  will  be  a  vacuum. 

The  column  will  be  33  ft.  9  in.  if  the  experiment  is 
performed  at  the  level  of  the  sea,  but  less  if  at  an  ele- 
vation. 

This  is  what  is  erroneously  called  "  suction." 


133.  Atmospheric  pressure  sustains  dif- 
ferent liquids  at  different  heights.— If   the 

above  experiment  be  tried  with  liquids  of  different 
densities,  the  heights  to  which  the  columns  will  rise 
are  inversely  as  their  specific  gravities.  Mercury,  for 
example,  is  13J  times  heavier  than  water;  hence,  33  ft.  9  in.  is  to 


102 : 


PNEUMATICS. 


FIG.  31. 


height  of  mercury  as  sp.  gr.  mercury  (13J)  is  to 
sp.  gr.  water  (1).  Or,  reducing  33  ft.  9  in.  to 
inches,  we  have, 

405  inches  is  to  height  of  mercury  as  13J  is  to  1 ; 
or,  (405  X  1)  -r  13J  =  30  (inches), 

as  the  height  to  which  mercury  would  rise  in  the 
experiment.  Many  practical  tests  have  proved 
that  this  is  the  average  height. 

Atmospheric  pressure  varies  at  the 
same  place. — It  is  found,  however,  that  col- 
umns of  water  or  mercury  do  not  always  stand 
at  the  same  height  at  the  same  place,  showing 
that  the  pressure  of  the  atmosphere  varies  at  the 
same  place.  (See  next  figure.) 


The  Barometer. 

134-  Figure  31.— The  barometer  and 
its  uses. — The  construction  and  operation  of 
this  instrument  depend  upon  the  principles  of  at- 
mospheric pressure,  Fig.  30  (132).  Take  a  glass 
tube,  about  34  inches  long,  open  at  one  end  and 
closed  at  the  other;  fill  it  with  pure  mercury, 
place  the  finger  over  the  open  end,  invert  the 
tube,  place  the  open  end  in  a  cup  of  mercury, 
remove  the  finger,  and  the  barometer,  essentially, 
is  made,  as  shown  in  the  figure.  The  mercury 
will,  at  first,  vibrate  up  and  down,  and  come  to 
rest  (if  the  weather  be  fair)  at  an  elevation  of  30 
inches,  when  at  the  level  of  the  sea. 

As  the  mercury  is  sustained  in  the  tube  only 
by  the  pressure  of  the  atmosphere,  whatever  af- 
fects this  pressure  will  vary  the  height  of  the 
mercurial  column.  As  the  pressure  of  fthe  air 
depends  upon  its  depth,  the  mercury,  of  course, 
will  fall  as  it  is  elevated  above  the  level  of  the  sea. 
Many  experiments  having  proved  the  amount  of 
its  fall  for  different  elevations,  it  has  become  a 
convenient  instrument  for  taking  altitude,  as  of 
mountains,  balloon  ascensions,  etc.,  and  also  for 
testing  the  pressure  of  the  atmosphere  at  different 


PNEUMATICS.  103 

times  and  places,  and  for  indicating  approaching  changes  of  weather, 
etc. 

135.  Height  of  the  mercury  at  different  elevations.— 

At  the  level  of  the  sea  it  stands  at  30        inches. 

5,000  ft.  above  the  sea  it  stands  "  24.773      " 

10,000  "          "  «  [height  of  Mt.  JEtna]  "  20.469      " 

15,000  «          "  "  [height  of  Mt.  Blanc]  "  16.896      " 

3  miles    "  "  "  16.36        •' 

6     "        [above  the  loftiest  mountains]  "     8.923      " 

9      "  «     4.866      " 

15      "  «     1.448      " 

136.  Barometer  as  a  weather-glass. — When  the  air  is  moist 
or  filled  with  vapors,  it  is  lighter  than  usual,  which  causes  the  mercury 
to  stand  low ;  but  when  the  air  is  dry  and  free  from  vapor,  it  is  heavier, 
and  supports  a  longer  column  of  mercury.     The  barometer,  therefore, 
generally  stands  high  in  fair,  and  loiv  in  foul  weather. 

Rules  for  reading  the  changes  of  the  barometer. — 1.  Sud- 
den falling  of  the  mercury  is  followed  by  high  winds  and  storms,  the 
mercury  sinking  lowest  when  the  wind  approaches  from  the  south. 

2.  Sudden  rising  of  the  mercury  indicates  coming  fair  weather. 

3.  A  fluctuating  and  unsettled  condition  of  the  mercurial  column 
indicates  changeable  weather. 

4.  If  the  mercury  falls  slowly,  a  long  continuation  of  foul  weather 
may  be  looked  for.     If  it  rises  slowly,  continued  fair  weather  may  be 
expected. 

5.  In  sultry  weather  the  falling  of  the  mercury  indicates  coming 
thunder.     In  winter  the  rising  of  the  column  indicates  frost.     In  frosty 
weather  its  fall  indicates  thaw,  and  its  rise  indicates  snow. 

For  convenience  of  noting  the  variations  in  the  barometer,  a  grad- 
uated scale  is  attached  to  the  upper  part  of  the  tube,  as  shown  in  the 
diagram  (31);  D  indicating  dry  weather;  F,  fair;  and  E,  rain.  Op- 
posite the  letters  are  figures,  showing  the  height  of  the  mercury. 

137.  Diurnal  variations  of  the  barometer.— The  mercury 

also  rises  and  falls,  slightly,  daily.  At  the  equator  the  maximum 
height  corresponds  to  9  o'clock  in  the  morning,  and  the  minimum 
height  to  4  o'clock  in  the  afternoon;  and  it  is  highest  again  at  9 
o'clock,  P.M.,  and  lowest  at  4  o'clock,  A.M. 

Capillarity  and  changes  of  temperature  must  be  taken  into  consider- 
ation in  making  close  observations  with  the  barometer. 


!04  PNEUMATICS. 

138.  Figure  32. — The  wheel  barometer. — In  the  common 
barometer,  the  rise  and  fall  of  the  mercury  is  indicated  by  a  scale  of 
inches  and  tenths  of  inches,  fixed  behind  the  tube  (Fig.  31) ;  but  it 
has  been  found  that  slight  variations  iii  the  density  of  the  atmosphere 
are  not  readily  perceived  by  this  method;  yet  it  is  desirable,  many 
times,  to  note  these  minute  changes.     The  object,  therefore,  of  the 
wheel  barometer  is  to  make  the  rise  and  fall  of  the  mercury  more 
sensible. 

The  tube  is  bent  at  the  bottom,  as  shown  in  the  figure,  and  in  its 
short  arm,  on  the  mercury,  is  placed  a  float,  L,  to  which  is  attached  a 
cord,  passing  over  a  pulley,  having  a  weight,  T,  fastened  at  the  other 
end.  As  the  mercury  rises  and  falls  in  the  long  arm  of  the  tube,  the 
float  also  rises  and  falls ;  which  communicates  motion  to  the  pulley. 
To  the  pulley  is  attached  an  arrow-pointer  that  rotates  in  front  of  a 
graduated  circular  disk,  as  shown. 

Of  course,  the  motion  of  this  pointer  will  be  as  much  greater  than 
that  of  the  mercury  in  the  tube,  as  the  circular  disk  is  greater  than 
the  pulley. 

On  the  outer  portion  of  the  disk  are  printed  (not  shown)  the  different 
conditions  of  weather,  to  correspond  with  the  movements  of  the  pointer. 

Changes  in  the  weight  of  the  atmosphere,  hardly  perceptible  by  the 
ordinary  barometer,  will  become  quite  apparent  by  this  instrument. 

139.  Figure  33. — Density  of  the  atmosphere  at  different 
altitudes. — Suppose  the  whole  height  of  the  atmosphere  to  be  forty- 
five  miles,  as  indicated  by  the  figures  and  graduated  scale  on  the  left  of 
the  diagram,  then  its  relative  density,  at  different  altitudes,  will  cor- 
respond to  the  relative  distances   between  the  horizontal  parallel  lines. 
The  relative  pressure  or  weight  at  different  altitudes  is  shown  by  the 
numbers  on  the  right  of  the  figure.     If  it  be  30  half  pounds  at  the  level 
of  the  sea,  then,  at  the  tops  of  the  highest  mountains  (or  about  five 
miles)  it  will  be  between  20  and  10  half  pounds  (say  about  12),  and  at 
20  miles  elevation  it  will  be  but  about  1  half  pound. 

The  whole  amount  of  air,  therefore,  above  the  altitude  of  nineteen 
miles,  equals  only  the  amount  contained  between  the  earth  and  the 
lowest  horizontal  line. 

Thus  it  is  seen  that  the  difference  in  the  density  of  the  atmosphere, 
at  different  altitudes,  is  very  great,  rapidly  diminishing  as  the  altitude 
increases,  which  is  due  to  the  iveiglit  and  extreme  compressibility  of  the 
air. 

The  weight,  and  consequent  pressure  of  the  atmosphere  upon  bodies 
near  the  surface  of  the  earth  and  upon  the  earth  itself,  was  not  gener- 
ally known,  until  Torricelli  first  announced  it  in  1643,  notwithstanding 


PNEUMATICS. 


105 


FIG.  32. 


FIG.  33. 


106 


PNEUMATICS. 


valuable  use  had  been  made  of  at- 
mospheric pressure  for  many  centu- 
ries, as  in  pumps,  siphons,  etc. 


FIG.  34. 


Figure  34.—  Balloons. 

—  Bodies  in  air  (like  solids  plunged 
in  liquids)  lose  as  much  of  their 
weight  as  equals  the  weight  of  the 
air  displaced.  Hence,  if  a  body 
weighs  less  than  an  equal  volume 
of  air,  it  will  rise  in  the  atmosphere 
until  it  meets  with  air  of  its  own 
density.  This  is  the  principle  upon 
which  heated  air,  smoke,  etc.,  rise. 

The  buoyant  effort  of  a  balloon 
of  a  given  size,  will  depend  upon 
the  lightness,  or  specific  gravity,  of 
the  gas  employed  to  fill  it.  Hydro- 
gen gas  is  usually  employed.  For 
convenience,  common  burning  gas 
is  used,  though  it  is  several  times  heavier. 

FIG.  35.  The  balloon  must  not  be 

completely  filled,  otherwise 
the  expansion  of  the  gas  is 
liable  to  burst  it,  as  the 
pressure  of  the  atmosphere 
diminishes. 

When  the  aeronaut  wish- 
es to  descend,  he  opens  a 
valve  by  means  of  a  cord, 
in  the  upper  part  of  the 
balloon,  to  allow  the  gas  to 
escape  ;  to  ascend,  he  throws 
out  ballast. 

1^1.  Figure  35.  - 
Diving-bell.  —  As  the  bal- 
loon is  the  means  of  ascend- 
ing into  the  air,  so  the 
diving-bell  is  the  means  of 
descending  into  the  water. 
It  consists  of  a  heavy  invert- 
ed vessel  of  suitable  size  and 
shape  —  usually  bell-shape. 


PNEUMATICS.  107 

Within  are  seats  upon  which  the  diver  sits  while  the  bell  is  being 
lowered  into  the  water,  by  means  of  a  rope  fastened  into  the  eye  at  the 
top. 

The  atmosphere  in  the  upper  portion,  K,  though  compressed  by  the 
upward  pressure  of  the  water,  furnishes  air  for  the  diver  to  breathe  ; 
though  only  for  a  short  time.  By  means  of  the  two  pipes,  A  and  B, 
extended  up  to  the  surface  of  the  water,  the  foul  air  is  removed  and 
fresh  air  supplied  ;  as  indicated  by  the  arrows. 

The  diving-bell  affords  another  illustration  of  the  impenetrability 
and  compressibility  of  the  air. 


.  Figure  36.—  Atmospheric  pressure  shown  by  in- 
verted tumbler  of  water.  —  If  a  tumbler  be  filled  with  water  and 
covered  with  paper,  and  then  inverted,  the  water  will  not  fall  out, 
owing  to  the  upward  pressure  of  the  air  on  the  paper. 

FIG.  36.  FIG.  37. 


'•  Figure  37.— Atmospheric  pressure  shown  by  cur- 
rents of  air. — Let  E  and  F  be  two  circular  disks  of  card-paper, 
about  the  size  of-  the  diagram,  having  a  quill  or  other  small  tube,  H, 
passing  through  the  centre  of  the  upper  disk,  as  shown.  Place  the 
upper  disk  parallel  to,  and  about  a  quarter  of  an  inch  above,  the  lower 
disk.  By  blowing  through  the  tube,  the  lower  disk  will  leap  up  and 
adhere  to  the  upper  one ;  and  the  harder  the  blowing,  the  firmer  it  will 
stick.  If  the  apparatus  be  inverted,  the  card  cannot  be  blown  off. 
The  reason  of  this  is,  that  the  blowing  more  or  less  drives  the  air  out 
from  between  the  disks,  which  diminishes  the  pressure  of  the  air  on 
their  inner  surfaces ;  thus  allowing  them  to  be  forced  together  by  the 
balance  of  atmospheric  pressure  without,  as  indicated  by  the  system 
of  arrows. 


108 


PNEUMATICS. 


144.  Figure  38. 
FIG.  38. 


Atmospheric  pressure  shown  by  tubes 
and  water. — If  a  tube,  L,  open  at  both  ends, 
be  vertically  sunk  in  water,  and  then  the 
thumb  placed  over  the  upper  end,  it  can  be 
lifted  full  of  the  liquid ;  owing  to  the  down- 
ward pressure  of  the  air  within  the  tube 
being  excluded,  and  thus  allowing  its  pres- 
sure on  the  outside  to  drive  the  water  into 
the  tube. 


145 '•  Figure  39.  —  Vacuum    foun- 
tain, showing  atmospheric  pressure. 

— Let  A  be  any  shaped  glass  vessel,  provided 
with  a  short  pipe  and  faucet,  and  means  for 
connecting  it  with  the  air-pump.  Exhaust 
the  air,  close  the  faucet,  remove  it  from  the 
pump,  insert  the  pipe  in  a  dish  of  water,  open  the  faucet,  and  the  down- 


FIG.  39. 


FIG.  40. 


ward  pressure  of  the  air  on  the  water  in  the  dish  will  drive  it  into  the 
vacuum  with  great  force,  as  shown  in  the  figure.     The  vacuum  simply 


PNEUMATICS. 


109 


removes  the  downward  pressure  of  the  air  from  the  water  in  the  jet- 
opening  of  the  pipe.     The  ball,  V,  will  be  alluded  to  hereafter  (149). 

146.  Figure  40.—  Animal  respiration  dependent  upon 
atmospheric  pressure.—  Water  is  not  raised  in  pumps  by  "  suc- 
tion," nor  do  we  breathe  by  drawing  or  "  sucking  air  "  into  our  lungs. 
Let  E  and  F  be  two  air-tight  sacks  or  bladders,  situated  in  glass 
jars,  and  communicating  with  the  external  air  ;  A  and  B,  leather 
caps,  tightly  fastened  to  the  jars. 

Now,  if  the  cap,  B,  be  drawn  down  (as  shown  at  A),  a  partial  vacuum 
will  be  formed  within  the  jar,  and  external  air  will  rush  in  at  H,  and 
distend  the  bladder,  F  (as  shown  at  E).  This  is  equivalent  to  inspira- 
tion. If,  now,  the  cap,  A,  be  forced  up  into  the  jar  (as  seen  at  B),  the 
air  in  the  bladder,  E,  will  be  forced  out  at  N,  leaving  the  bladder  col- 
lapsed (as  seen  at  F).  This  is  equivalent  to  expira- 
tion. 

The  jars  take  the  place  of  the  chest  ;  the  caps,  A 
and  B,  the  diaphragm  ;  the  bladders,  E  and  F,  the 
lungs  •  the  apertures,  H  and  N,  the  mouth.  The 
upward  and  downward  motions  of  the  ribs  aid  the 
process. 


FIG.  41. 


.  Figure   41.  —  Mariotte's   law,   re- 
lating to  the  elastic  force  of  gases.  —  The 

elastic  force  of  any  given  amount  of  gas,  whose 
temperature  remains  the  same,  varies  inversely  as 
its  volume.  Hence  it  follows  that  (if  the  tempera- 
ture remains  constant),  the  elastic  force  varies  as  the 
density. 

Pour  mercury  into  the  bent  tube,  A,  just  sufficient 
to  fill  the  bend  at  the  bottom  ;  then  the  air  in  both 
arms  will  be  alike  compressed  ;  that  is,  by  its  own 
weight  (15  pounds  to  the  square  inch).  If,  now, 
the  long  arm  be  filled  with  mercury  until  it  comes 
to  stand  30  inches  higher  than  in  the  short  arm, 
the  air  in  the  short  arm  will  be  submitted  to  the 
pressure  of  two  atmospheres  (30  pounds  to  the 
square  inch),  or  double  as  much  as  before  ;  which 
compresses  it  into  half  the  space  it  before  occupied, 
indicated  by  the  dotted  line  N.  That  is,  with  twice 
the  pressure  we  have  half  the  volume  ;  with  three 
times  the  pressure,  one-third  the  volume;  and  so 
on.  Or,  if  half  the  pressure  be  removed,  the  volume 


110 


PNEUMATICS. 


will  be  doubled,  and  so  on.  By  this  law,  at  a  pressure  of  814  atmo- 
spheres, air  would  become  as  dense  as  water. 

148.  Figure  42.— Condenser,  and  condensed  air.— Let  W 

be  the  vessel  in  which  the  air  is  to  be  condensed,  provided  with  a 
faucet.  The  balance  of  the  figure  represents  the  condenser,  which  is 
a  cylinder,  with  an  opening,  L,  at  the  top,  and  an  outward  working 
valve,  T,  at  the  bottom,  having  a  tightly-fitted  piston. 


FIG.  42. 


FIG.  43. 


If  the  piston  be  drawn  up,  the  expansion  of  the  air  in  the  vessel,  W, 
will  close  the  valve,  T,  and  the  air  in  the  cylinder  will  pass  through 
the  valve  in  the  piston.  If  the  motion  of  the  piston  be  reversed,  the 
air  below  it  will  be  compressed  and  driven  through  the  valve,  T,  into 
the  condenser,  while  the  cylinder  will  be  refilled  through  the  open- 


PNEUMATICS.  Ill 

ing,  L.  By  this  means  the  air  can  be  condensed  to  any  desired  extent. 
The  mercurial  tube  shows  the  degree  to  which  the  condensation  is 
carried,  and  operates  upon  the  principle  explained  in  the  last  paragraph. 

149.  Figure  43.— Condensed-air  fountain.— Fill  the  vessel 
here  shown  about  three-quarters  full  of  water,  and  then,  by  means  of 
the  condenser  just  described,  condense  the  air  above  the  water  (through 
the  pipe  and  faucet  F),  then  open  the  faucet,  H,  and  the  expansive  force 
of  the  compressed  air  will  act  upon  the  water,  and  drive  it  up  through 
the  .tube,  as  shown. 

The  ball  or  ring,  W,  on  the  side  of  the  stream,  is  held  up  against 
gravity  by  the  upward  force  of  the  jet,  and  at  that  point  where  the 
velocity  of  the  stream  equals  the  force  of  gravity.  The  ball  is  crowded 
against  the  stream  by  the  unequal  lateral  pressure  of  the  air ;  which  on 
the  side  of  the  jet  is  less  than  on  the  opposite  side  ;  owing  to  the  mo- 
tion of  the  water  driving  the  air  away  from  this  pIG  44 
side,  and  so  somewhat  diminishing  the  pressure; 
which  is  proved  by  the  fact  that  the  same  ball  or 
ring  will  not  rise  on  the  same  stream  in  a  vacuum, 
as  seen  in  figure  39  (145). 

150.  Figure  44. — Air-gun.— Air    is   con- 
densed in  the  hollow  globe,  T,  and  this  is  so  at- 
tached to  the  gun,  that  by  means  of  the  lock  of  the 
gun,  the  valve,  L,  is  opened,  and  the  air  is  thus 
instantaneously  allowed  to  escape  behind  the  ball, 
which  throws  it  out  with  great  force. 

The  velocity  of  the  ball  will  depend  on  its  size, 
and  on  the  density  of  the  air  in  the  magazine,  T. 
When  the  bore  of  the  gun  is  no  more  than  half  an 
inch,  or  so,  in  diameter,  it  is  estimated  the  ball  will 
have  a  force  not  much  less  than  that  of  a  musket- 
shot. 

As  these  guns,  in  their  discharge,  make  no  re- 
port, they  become  dangerous  secret  weapons  in  the 
hands  of  the  assassin,  and  therefore  their  use  is 
generally  prohibited  by  law. 

For  practical  use,  the  breech  of  the  gun  is  made 
of  strong  copper  plate,  and  constitutes  the  maga- 
zine, which  is  far  more  convenient  than  the  copper 
globe,  T ;  while  the  barrel  may  serve  as  the  tube 
of  the  condenser. 


112  HYDRA  ULIC8. 


CHAPTEK     VIII. 

(CHART  NO.  3.) 

HYDRAULICS. 
General  Principles. 

151.  Definition.— Hydraulics  is  that  part  of  hydro-dynamics 
which  treats  of  liquids  in  motion,  or  their  flow  and  elevation — espe- 
cially of  water — and  the  construction  of  all  kinds  of  instruments  for 
moving  them,  and  to  be  moved  by  them. 

'152.  Shape  of  orifices. — All  other  conditions  being  the  same, 
the  greatest  amount  of  water  will  flow  through  an  orifice  when  its 
length  is  twice  its  diameter. 

153.  Friction  between  liquids  and  solids. — The  central 
part  of  a  stream  in  a  pipe  flows  faster  than  that  next  to  the  pipe,  thus 
showing  there  is  friction  between  liquids  and  solids.  Hence,  pipes  for 
conveying  water  should  be  smooth  as  practicable.  Sudden  turns  in 
pipes  are  partial  obstructions  to  the  rapid  flow  of  water,  and  should 
be  avoided  when  possible  (190).  When  a  given  quantity  of  water  is 
required,  considerable  allowance  must  be  made  for  friction.  Though 
the  capacities  of  an  inch  and  a  two-inch  pipe  are  as  1  to  4,  yet  it  is 
found,  in  practice,  that  they  are  as  1  to  6,  if  their  lengths  be  100  feet. 

154-  Figure  1. — Velocity  and  gravity. — If  a  vessel  be  filled 
with  water,  and  three  openings,  A,  E,  F,  made  at  different  heights, 
the  water  will  be  driven  out  by  the  lateral  pressure  of  the  liquid ;  and 
as  the  pressure  depends  upon  the  height  of  the  liquid  in  the  vessel,  of 
course  the  lower  the  orifice  the  greater  the  velocity  of  the  stream  and 
the  amount  of  the  discharge.  These  streams,  being  acted  upon  by 
gravity,  take  the  curves  resulting  from  the  two  forces  (62). 

The  projectile  force  varying  with  the  height  of  the  liquid,  the 
curves  will  not  be  alike.  The  fluid,  therefore,  obeys  the  same  laws 
that  solids  do  when  projected,  and  falls  in  curved  lines  depending  on 
the  velocity  (62). 

The  jet  E,  flowing  from  half  the  height  of  the  fluid,  has  the  greatest 
possible  horizontal  range,  and  all  jets  made  equally  distant  above  and 
below  this  orifice,  as  A  and  F,  will  have  equal  horizontal  range  with 
each  other. 


HYDRAULICS. 
FIG.  1. 


113 


155.  Figure  2.— Velocity  of  discharge. — The  velocity  of 


flow  of  liquids  from  an  orifice  is  as  the 
square  root  of  the  head.  Let  the  vessel  be 
graduated  into  25  equal  spaces  (say  inches) 
on  the  left  side,  as  shown. 

Opposite  the  1st  division  set  1,  the  square 

root  of  1. 
Opposite  the  4th  division  set  2,  the  square 

root  of  4. 
Opposite  the  9th  division  set  3,  the  square 

root  of  9. 
Opposite  the  16th  division  set  4,  the  square 

root  of  16. 
Opposite  the  25th  division  set  5,  the  square 

root  of  25. 

Streams  flowing  from  this  vessel,  where 
their  velocities  would  be  to  each  other  as 
1,  2,  3,  4,  and  5,  would  start  opposite  the 
divisions  indicating  the  depth  of  the  fluid 
1,  4,  9,  16,  and  25.  Or,  in  tabular  form : 

8 


FIG.  2. 


114 


HYDRAULICS. 


Velocity |  1  |   2   |  8  |  4   |   5   |  6   |   7   |   8   |   <J   |    10   |   11 


Depth I  1  I  4 


li. 

25  |  b<>  |  49  |  64  |  81  |  100  |  121  |  144 


This  is  the  same  as  the  law  of  falling  bodies  (55).  Hence,  the  ve- 
locity of  discharge,  at  any  orifice,  is  the  same  as  the  velocity  of  a  body 
falling  freely  through  a  height  equal  to  the  depth  of  the  centre  of  the 
orifice  below  the  surface  of  the  fluid. 


Figure  3.  —  Flowing  of  rivers.  —  Owing  to  the  friction 
between  fluids  and  solids,  the  sides  and  bottom  of  a  river  flow  less 
rapidly  than  the  central  and  upper  parts  of  the  stream.  This  is  shown 
in  the  figure  by  a  weighted  stick,  floating  in  a  river,  with  the  top,  H, 
leaning  in  the  direction  of  the  current,  whereas  in  still  water  it  would 
stand  upright.  The  arrow-heads  show  the  direction  of  the  current. 


FIG.  3, 


FIG.  4. 


157.  Figure  4.— Finding  the  velocity  of  rivers. — This  is 
done  by  stationary  revolving  wheels,  and  floating  bodies,  and  also  by 
means  of  a  tunnel-shaped  tube,  open  at  both  ends  and  bent  at  right 
angles,  as  shown  in  the  figure.  The  current,  W,  and  its  direction,  are 
indicated  by  the  horizontal  lines. 

The  tube  is  placed  in  the  water  with  its  mouth  toward  the  current, 
and  the  rapidity  of  the  stream"  is  estimated  by  the  height  to  which  the 

water  is  forced  into  the  tube,  N,  above  the  surface  of  the  river. 

- 


HYDRA  ULICS. 


Water  as  Motive  Power. 


115 


158.  Many  devices  have  been  invented  for  utilizing  the  fall  or 
gravity  of  water  as  a  motive  power.     The  principles  involved  in  those 
most  extensively  employed  are  illustrated  and  briefly  described. 

Though  various  reciprocating  and  rotary  water-engines,  similar  to 
steam-engines,  have  been  used,  the  most  simple  and  common  way  of 
applying  the  fall  of  water  is  by  means  of  various  kinds  of  wheels,  called 
wafer-wheels;  which  maybe  divided  into  four  classes,  as  represented 
by  the  following  illustrations. 

159.  Figure  5.— Overshot  water-wheel.— The  water  falls 
from  the  canal,  W,  upon  the  top  of  the  wheel,  and  fills  the  buckets, 

FIG.  5. 


TF,  which  are  pitched  toward  the  stream,  and  hold  the  water  until 
they  reach  the  position  F,  where  they  begin  to  discharge.  This  is  the 
most  powerful  of  all  the  water-wheels  of  similar  construction,  and  is 
moved  principally  by  the  gravity,  and  slightly  by  the  momentum,  of 
the  water.  L  is  a  gate  for  shutting  off  the  stream.  The  operation  of 
this  wheel  is  too  evident  to  need  further  description.  Water  has  the 
greatest  effect  on  this  wheel  when  its  buckets  move  about  three  feet 
per  second. 


116 


HYDRAULICS. 


160.  Figure  6.— Breast  water-wheel. — This  wheel  receives 
the  water  at  about  half  its  own  height,  and  is  moved  both  by  the  weight 

FIG.  6. 


FIG.  7. 


and  momentum  of  the  water.  It  is  furnished  either  with  buckets  or 
float-boards,  TL,  fitting  the  water-course.  N  is  the  gate  for  shutting 
off  the  water. 

161.  Figure  7.  - 
Undershot  water- 
wheel.— This  is  provided 
with  vanes,  or  float-boards, 
projecting  from  its  peri- 
phery into  the  stream,  as 
shown,  and  is  moved  prin- 
cipally by  the  momentum 
of  the  water ;  requiring  no 
fall,  but  a  rapid  flow  of 
the  stream.  To  yield  the 
greatest  effect,  the  floats 
in  this  wheel  should  move 
about  five-twelfths  as  fast 
as  the  water.  This  wheel 
is  extensively  employed 
for  elevating  (166). 


HYDRAULICS. 


117 


162.  Figure  8.— Turbine  water-wheel. — The  turbine,  of 
which  there  are  many  modifications,  is  a  horizontal  wheel.  It  works 
submerged,  and  is  the  most  energetic  and  economical  of  all  the 
water-wheels;  some  of  them  having  utilized  eighty-eight  per  cent, 
of  the  entire  power  of  the  water.  They  are  applicable  to  large  and 
small  streams,  and  great  and  small  falls  of  water.  W  is  the  wheel, 
N  the  guides,  to  change  the  direction  of  the  water  so  it  shall  strike 
the  flanges  of  the  wheel  at  the  most  advantageous  angle.  T  is  the  shaft 
which  carries  the  wheel  and  the  driving  pulley,  F.  The  water  falls  ver- 
tically upon  these  stationary  guides,  which  change  its  direction  (as 
shown  by  the  long  arrows)  so  it  shall  fall  upon  the  flanges  of  the  wheel 
nearly  at  right  angles  to  their  faces  (as  indicated  by  the  points  of  the 
arrows) ;  when,  by  reaction  on  the  blades  of  the  wheel,  its  course  is 
again  changed,  and  it  passes  out  below  the  wheel,  as  shown  by  the 
short  arrows. 


FIG.  8. 


FIG.  9. 


163.  Figure  9.— Reaction  and  centrifugal  machine,  or 
Barker's  Mill.— H  is  an  upright  cylinder,  funnel-shaped  at  the  top, 
to  receive  the  stream  of  water  from  the  pipe,  W— standing  on  a  point 
and  held  in  position  by  a  projecting  spindle  at  the  top ;  the  arms,  T, 
are  tubes  communicating  with  the  cylinder.  In  the  tubes  are  open- 
ings from  which  the  water  issues.  These  openings,  removing  a  portion 
of  the  internal  surface  of  the  tubes,  destroy  the  equilibrium  of  the  lat- 
eral pressure  of  the  fluid  within,  and  set  the  cylinder  to  moving,  with 
great  velocity,  in  the  direction  of  the  resultant  force  of  pressure;  that 


118  HYDRAULICS. 

is,  in  the  opposite  direction  to  the  flow  of  the  jets.     The  cylinder  must 
be  kept  full  of  water. 

The  remarkable  feature  about  the  operation  of  this  machine  is  its 
great  velocity ;  which  is  caused  in  part  by  the  centrifugal  force  of  the 
water  in  the  tubes,  which,  of  course,  greatly  increases  the  reactionary 
force,  and  this  again  increases  the  speed,  which  again  further  increases 
the  centrifugal  force,  and  so  on ;  each  force  increasing  the  other. 

Machines  for  Elevating  Water. 

16 4-  Variety  of  water-elevators. — In  the  earliest  ages  there 
wer,e  devices  for  elevating  water,  and  every  subsequent  age  has  added 
new  ones,  until  machines  for  this  purpose  are  almost  without  number 
— -volumes  would  be  required  to  describe  them.  Yet,  in  this  country, 
as  well  as  others,  some  of  the  earliest  and  crudest  of  these  are  still  em- 
ployed, as  the  well-sweep,  windlass,  and  rope  and  pulley.  In  fact,  few, 
if  any,  machines  have  undergone  a  greater  number  of  metamorphoses 
than  that  class  of  these  devices  termed  pumps. 

Only  a  few  of  the  ancient  and  some  of  the  best  of  the  modern  con- 
trivances for  elevating  water  are  here  described. 

FIG.  10. 


165.  Figure  10.— Lifting- wheel. — This  is  a  wheel  consisting 
of  a  series  of  hollow  qr  tubular  spokes,  bent  at  right  angles  at  the  ex- 
tremities, to  form  cups  or  buckets,  TL.  The  wheel  is  set  so  that  when 
it  is  revolved  by  hand  or  other  power,  these  cups  will  be  filled  with  the 
water  to  be  elevated.  As  each  cup  rises  to  the  level  of  the  axis,  the 


HYDRAULICS.  119 

water  will  run  through  its  spoke  to  the  centre,  F,  of  the  wheel,  where 
it  is  discharged,  as  shown  in  the  diagram. 

If  floats  or  disks  were  fastened  to  the  extremities  of  the  spokes,  the 
current  of  a  stream  would  revolve  the  wheel. 

106.  Figure  11.— Wheel  and  buckets,  or  Persian  Wheel. 

— This  consists  of  a  series  of  swinging  buckets,  fastened  to  the  rim  of 
mi  undershot  water-wheel  (161),  as  seen  in  the  diagram.  As  the  wheel 
is  revolved  by  the  stream,  the  buckets  are  filled  as  they  pass  into  the 

FIG.  11. 


water ;  carried  up,  and  overturned  by  coming  in  contact  with  a  pin  in 
the  trough.  Arrows  in  the  water-lines  show  the  direction  of  the  cur- 
rent and  the  motion  of  the  wheel. 

This  is  an  ancient  Persian  invention.  The  greatest  work  in  France, 
for  artificial  irrigation,  was  a  series  of  these  wheels,  in  Languedoc, 
raising  water  thirty  feet.  They  are  still  used  in  various  parts  of  Europe 
and  Asia  for  supplying  cities  with  water,  irrigating  land,  etc.  In 
Hamath,  an  ancient  city  of  Syria,  celebrated  for  its  water-works,  these 
wheels  are  employed;  some  of  them  being  seventy  feet  in  diameter. 

The  construction  of  the  water-works  of  Hamath  have  remained  un- 
altered, in  their  general  design,  from  very  remote  times.  The  peculiar 
locality  of  the  river  (named  El  Ausi,  the  swift),  and  its  consequent 
adaptation  to  undershot  wheels,  render  it  probable  that  the  present 
mode  of  raising  water  is  much  the  same  as  when  this  city  flourished 
under  Solomon. 


120 


HYDRAULICS. 


FIG.  12. 


FIG.  13. 


167.  Figure  12.  —  Endless 
chain  of  pots. — The  chain  of  pots 
is  an  ancient  invention,  and  is  used 
as  an  overshot  water-wheel,  where  the 
fall  of  water  is  great,  and  the  stream 
small;  though  it  is  chiefly  employed 
for  elevating  water,  cleaning  docks, 
deepening  harbors,  etc. 

The  chain  is  hung  on  arms,  T,  of  a 
wheel,  as  seen  in  the  diagram.  As  the 
wheel  is  revolved,  the  pots  rise  filled 
on  one  side,  and  descend  empty  on  the 
other ;  their  contents  being  discharged 
into  a  cistern  as  they  pass  over  the 
wheel.  The  direction  of  the  move- 
ment is  indicated  by  the  arrows  seen 
at  the  mouth  of  the  well,  W. 


168.  Figure  13. — Chain  pump. — This  consists  of  a  cylinder, 
E,  with  its  lower  end  standing  in  the  water  of  the  reservoir  or  well,  N, 

and  its  upper  end  terminating 
in  a  trough.  An  endless  chain 
is  carried  around  a  wheel,  H, 
above  and  below,  and  is  fur- 
nished, at  equal  distances,  with 
circular  disks  which  fit  closely 
in  the  cylinder. 

As  the  wheel  is  revolved 
(usually  by  a  crank)  the  disks 
successively  enter  the  cylinder 
and  carry  the  water  up  before 
them,  into  the  trough,  from 
which  it  is  discharged.  The 
arrows  indicate  the  direction 
of  the  movement. 

The  chain  pump  is  made  in 
various  forms.  In  China  the 
cylinder  or  trough  is  usually 
made  square,  and  often  inclined 
to  the  horizon.  Instead  of  the 
circular  disks  and  an  iron  chain, 
stuffed  globular  cushions  made 
of  leather,  and  attached  at  regu- 


HYDRAULICS. 


lar  intervals  to  a  rope,  have  been  substituted.  It  has  been  used  in  all 
countries,  and  is  extensively  employed  at  the  present  day.  It  had  its 
origin,  probably,  in  China,  many  centuries  ago. 

This  pump  seems  to  be  the  connecting  link  between  the  chain  of 
pots  and  the  ordinary  lifting  and  suction  pump  ;  hence,  in  connection 
with  the  history  of  hydraulic  devices,  the  place  and  date  of  its  origin 
are  matters  of  considerable  interest. 

In  their  various  forms,  the  Persian  wheel,  the  chain  of  pots,  and  the 
chain  pump,  are  now,  as  they  ever  have  been  in  all  Eastern  countries, 
among  the  principal  devices  for  elevating  water. 

169.  Figure    14.— First  invented  centrifugal   pump.— 

This  pump  was  invented  in  1732.    It  is  merely  u  straight  tube,  N, 

FIG.  14. 


attached,  in  an  inclined  position,  to  a  vertical  axis,  W,  and  whirled 
around  by  the  crank,  or  by  a  pulley,  H. 

As  the  motion  begins,  the  water,  by  centrifugal  force,  is  thrown  into 
the  tube  (as  indicated  by  the  arrow),  and  out  of  its  mouth,  as  shown. 
In  the  figure  the  vertical  axis  is  a  box,  open  at  the  bottom ;  but  a 
simple  shaft  of  wood  may  be  substituted  for  it. 


HYDRAULICS. 


170.  Figure  15.— The  T-centrifugal  pump. — This  pump 
consists  of  two  communicating  tubes,  united  in  the  form  of  the  letter 

FlG  15  T.     The  main  tube,  F,  has 

openings  at  the  bottom,  and 
stands  on  a  point,  being  held 
upright  by  a  stem  and  brace 
at  the  top,  and  has  a  valve, 
H,  in  the  bottom.  On  the 
top  is  a  cross-tube,  LL,  un- 
derneath which  is  a  circular 
trough,  TT,  to  receive  the 
discharged  water ;  and  above 
is  a  grooved  pulley  to  con- 
nect with  the  power. 

OPERATION. — Having  first 
filled  the  tubes  with  water, 
if  the  pump  be  started  by 
rotating  the  arms,  LL,  the 
water  contained  in  these  will 
be  thrown  out  by  centrifugal 
force,  and  the  atmospheric 
pressure  on  the  surface  of 
the  water  below  will  force 
the  liquid  up  through  the 
valve  to  keep  the  arms  sup- 
plied, as  shown  by  the  arrows.  The  water  is  delivered  from  the  trough 
through  the  apertures  T  and  N". 

171.  Figure   16.— Archimedes'   screw. — Wind  a  pipe  spi- 
rally around  an  inclined  cylinder,  provided  with  bearings  and  a  crank, 
H  ;  the  lower  end  being  immersed  in  a  reservoir,  so  the  end  of  the  pipe 
will  dip  into  the  water.    In  the  figure  there  are  two  pipes,  but  the 
principle  is  the  same. 

OPERATION. — Suppose  the  machine  to  be  at  rest,  and  a  metallic  ball 
dropped  into  the  pipe  at  T ;  it  is  plain  that  it  will  roll  down  to  the 
position  of  1,  and  remain  at  rest.  Suppose,  as  shown,  there  is  a  ball  in 
each  bend.  Now,  by  revolving  the  cylinder,  the  pipe  will  fall  in  front 
and  rise  behind  each  ball,  which  gives  each  of  them  a  forward  motion 
(that  is,  toward  the  handle)  ;  but  if  the  cylinder  be  revolved  the  other 
way,  they  will  have  a  backward  motion.  Hence,  by  one  revolution  the 
ball  1  would  move  forward  to  the  position  of  ball  3 ;  and  ball  2  to  that 
of  ball  4;  and  soon;  until  they  would  drop  out  of  the  upper  end 
of  the  pipes.  If  water  be  poured  (or  scooped)  in  with  the  balls, 


HYDRAULICS.  123 

it  will  go  along,  and  be  discharged  with  them,  as  indicated  in  the 
diagram. 

FIG.  16. 


Such  screws  are  employed  to  elevate,  besides  water,  ores,  grain,  etc. 
and  are  commonly  set  at  an  inclination  of  about  45°. 


.  Figure  17.—  Hydraulic  ram.—  In  this  machine  the  mo- 

mentum of  a  part  of  the  fluid  in  motion  is  effective  in  raising  another 

FIG.  17. 


portion  ;  and,  by  means  of  which,  a  large  stream,  with  small  fall,  will 
elevate  a  small  stream  to  a  great  height. 

OPERATION. — The  water  from  the  cistern  or  stream,  S,  runs  down 
through  the  pipe,  W,  and  out  at  the  aperture,  H,  until  the  velocity  of 
the  current  is  sufficient  to  lift  and  close  the  ball-valve,  L.  This  aperture 
being  closed,  the  momentum  of  the  water  is  suddenly  checked,  which 
drives  a  portion  of  the  liquid  through  the  valve  of  the  air-chamber,  N, 
where  it  compresses  the  air,  as  indicated  by  the  small  arrow.  The  elasti- 
city of  the  air  reacts  on  the  fluid,  closes  the  chamber-valve,  and  drives  the 


124 


HYDRAULICS. 


water  up  through  the  discharge  pipe,  F.  The  water  in  the  pipe,  W, 
having  now  come  to  a  state  of  rest,  the  ball-valve,  L,  by  its  weight, 
will  drop  down  and  again  open  the  aperture,  H,  when  the  same  opera- 
tion will  be  repeated,  and  so  on. 


Suction  Pumps. 

173.  Figure  18.— The  principle  of  suction  pumps. — The 

operation  of  suction-pumps,  so  far  as  regards  the  suction,  consists  in 
producing,  by  means  of  a  cylinder  and  piston,  or  other  device,  a  vacuum, 
which  becomes  filled  with  water,  by  the  downward  pressure  of  the 
atmosphere.  As  fast  as  the  piston  lifts  the  air  out  of  the  cylinder,  F, 


FIG.  18. 


FIG.  19. 


the  pressure  of  the  air  drives  the  water  in.  Or,  in  other  words,  a 
suction  pump  consists  of  a  tube  not  more  than  34  feet  long,  with  any 
sort  of  contrivance,  at  the  top,  that  will  remove  the  downward  pressure 
of  the  air  within,  so  that  its  pressure  on  the  water  without  will  force 
the  liquid  up  the  tube.  A  piece  of  straw  and  the  mouth,  therefore, 
are  a  suction  pump— being,  probably,  the  first  pump  known,  and  that 
which  led  to  the  invention  of  others. 


HYDRAULICS. 


125 


The  figure  represents  a  simple  cylinder  and  piston.  If  the  piston  be 
raised,  the  vacuum  formed  below  it  will  be  filled  with  water.  If  the 
piston  is  not  held  up,  the  downward  pressure  of  air  will  bear  it  down, 
and  the  water  in  F  will  return  to  the  cistern. 

Water  will  rise,  by  suction  or  atmospheric  pressure,  34  feet,  theoret- 
ically ;  practically,  it  should  be  rated  about  2  feet  less. 

For  conditions  of  the  atmosphere,  and  other  circumstances  affecting 
the  operation  of  this  principle,  see  131  and  132. 

174-  Figure  19.— Proof  of  atmospheric  pressure  in 
pumps. — To  prove  that  water  rises  in  suction  pumps  only  by  pressure 
of  air  on  the  fluid  outside  of  the  pump,  let  the  reservoir,  E,  which  sup- 
plies the  pump,  be  a  closed  vessel,  provided  with  a  faucet,  as  shown. 
If  the  faucet  be  closed,  which  shuts  off  the  air,  and  the  piston,  L,  drawn 
up,  the  water  will  not  follow  it ;  and  a  vacuum  will  exist  at  N.  If, 
now,  the  faucet  be  opened,  the  air  will  rush  into  the  reservoir,  and 
press  the  water  up  the  pipe  into  the  vacuum  at  N. 

Rotary  Pumps. 

175.  Figure  20. — Double-cylinder  rotary  pump. — There 
are  many  kinds  of  rotary  pumps  ;  of  which  three  are  here  represented. 
All  rotary  pumps  are  both  sucking  and  forcing  machines. 

FIG.  20. 


T  is  a  cylindrical  case  (with  head  removed) ;  A,  a  cylinder  consider- 
ably smaller  than  the  case,  provided  with  three  teeth  or  blades,  which, 


126 


HYDRAULICS. 


FlG 


acting  as  pistons,  work  water-tight  against  the  interior  surface  of  the 
case  ;  N,  a  small  cylinder  (termed  the  hutment),  revolving  against  the 
large  one,  and  provided  with  a  curved  indentation  on  one  side  to  allow 
the  blades  to  pass  ;  S,  the  supply-pipe  ;  H,  the  discharge-pipe  ;  and  at 
the  centre  of  the  cylinder  is  a  shaft  to  which  is  applied  the  power. 
The  arrows  indicate  the  direction  in  which  the  water  and  the  large 
cylinder  move. 

OPERATION.  —  The  lower  blade,  for  instance,  drives  the  water  before 
itself,  and  leaves  a  vacuum  behind,  which  is  filled  with  water  from  the 
supply-pipe.  The  blade,  T,  also  drives  the  water  in  front  of  itself;  and, 
as  the  liquid  cannot  pass  around  between  the  cylinders,  it  is  driven  up 
through  the  discharge-pipe  ;  while  the  indentation  in  the  small  cylinder 
allows  the  blades  to  pass  by  itself  without  causing  an  opening  for  the 
escape  of  the  water. 

176.  Figure  21.—  Single-cylinder  rotary  pump.—  In  this 
pump  is  a  solid  wheel,  T,  formed  into  three  spiral  wings,  L,  acting  as 

pistons,  and  turned  round  within  a 
cylindrical  case  (the  head  in  the  fig- 
ure being  removed).  The  hutment, 
F,  is  a  heavy  piece  of  metal,  working 
water-tight  through  a  stuffing-box, 
N,  and  as  wide  as  the  wings  ;  and, 
by  its  weigJtt,  bears  water-tight  on 
the  face  of  the  wings;  being  kept 
upright  by  passing  between  rollers 
(not  shown).  The  arrows  indicate 
the  motion  of  the  wheel  and  water. 
A  stuffing-box  is  a  contrivance  for 
making  a  tight-working  fit.  See 
Fig.  42,  Chart  4  (353). 

OPERATION.  —  As  the  wheel  is 
turned,  the  wings,  working  water- 
tight in  the  case,  drive  the  water 
before  them  through  the  valve  into 
the  discharge-pipe,  H  ;  and  the  hutment,  F,  rising  and  falling  on  the 
face  of  the  wings,  prevents  the  water  from  passing  around  with  the 
wheel.  The  water  is  admitted  into  the  case  through  openings  (not 
shown)  in  the  bottom.  The  valve  in  the  discharge-pipe  is  to  prevent 
the  water  from  returning  while  the  pump  is  at  rest. 


1  7  7.  Figure  22.  —  Double   cog-wheel   rotary  pump.— 

This  is  one  of  the  oldest  rotary  steam-engines,  but  now  principally 


HYDRAULICS.  127 

employed  as  a  suction  and  force  pump,  for  which  purpose  it  is  a  power- 
ful machine. 

It  consists  of  two  cog-wheels,  the  teeth  working  water-tight  into 
each  other,  and  against  the  interior  surface  of  a  cylindrical  case — the 
front  head  in  the  figure  being  removed.  It  is  worked  by  power  applied 

FIG.  22. 


to  the  shaft  of  one  of  the  wheels.    As  one  wheel  turns  the  other,  they 
revolve  in  opposite  directions. 

OPERATION. — As  the  wheels  revolve  (in  the  direction  of  the  arrows), 
a  portion  of  the  water  below  is  carried  between  their  teeth  and  the 
case,  as  at  N  and  L,  around  to  the  upper  side,  and  as  it  cannot  pass 
down  between  the  wheels,  is  forced  into  the  discharge-pipe  above,  as 
shown  by  the  double  arrow;  while  the  vacuum  thus  formed  belo\v  is 
filled  from  the  supply-pipe,  as  indicated  by  the  other  double  arrow. 

Many  volumes  would  be  required  to  describe  the  different  kinds  of 
rotary  pumps  which  have  been  invented,  yet  they  have  never  retained 
a  permanent  place  among  machines  for  raising  water.  Besides  being 
more  expensive  than  other  pumps,  they  are  too  complex  and  too  easily 
deranged  to  be  adapted  for  common  use.  Theoretically  considered 
they  are  perfect  machines,  but  practical  difficulties  render  them  (like 
rotary  steam-engines)  inferior  to  others.  In  cases,  however,  where 
strong  and  rapid  action,  instead  of  durability  and  saving  of  expense, 
are  the  leading  objects,  as  in  the  case  of  steam  fire-engines,  they  are 
employed  with  great  success. 


128 


HYDE  A  ULICS. 


178.  Figure  23.— The  bellows  suction  pump.— The  bel- 
lows pump  is  the  oldest  of  which  history  gives  any  account,  and  was 

probably  suggested  to  some  primi- 
tive inventor  by  sucking  water 
through  a  straw. 

It  consists  of  a  leather  bag,  H, 
the  lower  end  of  which  is  fastened 
water-tight  to  the  bottom  of  an 
open  dish,  and  the  upper  end  to  a 
board  or  disk  through  which 
there  is  an  aperture,  the  disk 
being  provided  with  a  suitable 
handle  and  an  outward  working 
valve,  as  shown. 

OPERATION. — If  the  handle  be 
drawn  up,  a  vacuum  will  be  formed 
within  the  bellows,  which  will  be 
filled  from  the  supply-pipe  below, 
as  indicated  by  the  arrow.  By 
reversing  the  motion,  the  valve 
over  the  supply-pipe  will  be  closed, 
and  the  water,  H,  in  the  bellows  will  lift  the  upper  valve  and  pass  out 
into  the  open  dish,  and  flow  off,  as  indicated  by  the  double  arrow  and 
spout. 

179.  Figure  24.  —  Dia- 
phragm suction  pump. — This 
consists  of  a  diaphragm,  N,  work- 
ing in  a  cylinder  or  open  dish.  The 
outer  edge  of  the  diaphragm,  E 
(made  of  leather),  is  fastened  water- 
tight to  the  interior  surface  of  the 
cylinder,  in  the  manner  shown,  on 
the  central  portion  of  which  is  se- 
cured a  disk,  N,  provided  with  an 
aperture,  handle,  and  outward 
working  valve. 

OPERATION. — If  the  diaphragm 
be  lifted  by  means  of  the  handle, 
its  valve  will  close,  and  the  water 
above  be  raised  and  discharged  at 
the  spout,  and  the  vacuum,  formed 
below,  filled  with  water  from  the 
cistern.  By  reversing  the  motion,, 


FIG.  24. 


HYDRA  UL1V&  j  3  g 

the  lower  valve  will  close,  and  the  water  below  the  disk  will  open  the 
upper  valve  and  pass  through  the  disk,  as  shown  by  the  arrow. 

180.  Figure  25.— Plunger,  force,  and  suction  pump.— 

This  is  chiefly  employed  for  lifting  or  forcing  small  quantities  of  water 
against  great  resistance,  as  for  feeding  steam- 
boilers. 

It  consists  of  a  cylinder,  the  usual  valves, 
and  a  plunger,  A,  in  place  of  a  piston,  which 
is  a  solid  bar  somewhat  longer  than  the  stroke 
of  the  pump.  The  plunger,  instead  of  coming 
in  contact  with  the  interior  surface  of  the  cylin- 
der, passes  through  a  stuffing-box,  L,  at  the  top 
of  the  cylinder,  as  shown  in  the  figure.  The 
power  is  applied  at  the  top  of  the  plunger. 

OpEKATiotf. — As  the  plunger  is  drawn  up, 
the  valve  in  the  discharge-pipe  will  be  closed, 
and  the  vacuum,  formed  within  the  cylinder, 
filled  with  water  rushing  through  the  lower 
valve  from  the  cistern.  If  the  motion  be  re- 
versed, the  lower  valve  will  be  closed,  and  the 
water  in  the  cylinder  forced  through  the  upper 
valve  into  the  discharge-pipe,  as  indicated  by 
the  arrow. 

181.  Figure  36.— Single-cylinder  suction  pump.— This  is 

the  most  common  of  all  pumps,  being 
employed  to  draw  water  from  house-cis- 
terns, and  consists  of  an  open  cylinder, 
piston,  and  the  two  usual  valves.  When 
the  cylinder  is  extended  much  above  the 
upper  valve,  the  water  above  this  valve 
being  lifted  instead  of  sucked,  it  is 
termed  the  suction  and  lifting  pump,  and 
in  this  form  is  used  for  wells,  which  are 
usually  too  deep  for  the  simple  suction 
pump.  The  upper  valve  is  placed  in  the 
piston,  and  the  lower  valve  at  the  botto.M 
of  the  cylinder.  In  the  ordinary  wood  n 
pump  the  bore  of  the  log  constitutes  l'ie 
cylinder,  and  extends  the  whole  length  of 
the  pump. 

OPERATION. — As  the  cylinder  descends 
9 


FIG.  26. 


1:30 


HYDRA  ULICS. 


the  lower  valve  closes,  and  the  water  above  it  passes  through  the  upper 
valve.  The  motion  being  reversed,  reverses  the  valves,  and  discharges 
the  water  above  the  piston,  while  the  vacuum,  thus  formed  below  it,  is 
filled  from  the  cistern  or  well. 

182.  Figure  27. — Suction  and  force  pump. — This   is  the 
pump  usually  employed  for  conveying  water  from  cisterns  to  upper 

FIG.  27. 


rooms,  sprinkling  door-yards,  etc.  The  upper  valve  is  placed  in  the 
mouth  of  an  air-chamber,  instead  of  the  cylinder.  The  object  of  the 
air-chamber  is  to  soften  the  action  of  the  pump  and  render  the  dis- 
charge continuous. 

OPERATION.— By  raising  the  piston  the  upper  valve  will  be  closed, 
and  the  vacuum,  formed  under  the  piston,  filled  with  water  from  the 
cistern  passing  through  the  lower  valve.  If  the  motion  is  reversed, 
the  lower  valve  closes,  and  the  piston  forces  the  water  beloAV  it  through 
the  upper  valve  into  the  air-chamber  faster  than  it  can  escape  through 
the  discharge-pipe;  and  the  air  above,  being  thus  compressed,  will,  by 
its  elasticity,  press  upon  the  water  (as  indicated  by  the  arrows)  and 
keep  up  a  discharge,  while  the  piston  is  refilling  the  cylinder. 


HYDRAULICS. 


131 


183.  Figure  28. — Double-acting  suction  and  force  pump. 

—'I'h is  consists  of  a  cylinder,  piston,  and  four  valves,  with  supply 
and  discharge  apertures  at  both  ends  of  the  cylinder. 

OPERATION. — The  cylinder  and  all  the  pipes  being  full  of  water,  if 
the  piston  be  drawn  up,  the  valve  N  will  close  and  prevent  the  return 
of  water  from  the  discharge-pipe,  and  the  valve  H  will  also  close  and 
prevent  the  water  above  the  piston  from  returning  to  the  cistern 
through  the  supply-pipe,  thus  causing  the  water  above  the  piston  to 
be  forced  through  the  valve  F  into  the  discharge-pipe  ;  and  the  valve 
E  will  be  opened  by  the  water  rushing  up  from  the  cistern  to  fill  the 
vacuum  formed  below  the  piston,  as  indicated  by  the  position  of  the 
valves  and  the  two  double  arrows.  If  the  movement  of  the  piston  be 
reversed,  it  will  reverse  all  the  valves,  and  the  cylinder  will  be  filled 
through  the  valve  H,  and  emptied  through  the  valve  N  ;  the  valve  F 
will  close  to  prevent  the  return  of  water  from  above,  and  the  valve  E 
will  close  to  prevent  the  water,  W,  below  the  piston  from  returning  to 
the  cistern. 

FIG.  28.  FIG.  29. 


184-  Figure  29.— Single-acting  suction  and  force  pump. 

-This  is  a  strong,  powerful  force  pump, usually  employed  where  force 


HYDRA  ULICS. 


FIG.  30. 


pumps  of  large  capacity  are  required,  as  in  large  low-pressure  steam- 
engines. 

Its  operation  is  the  same  as  the  one  illustrated  by  Fig.  25  (180), 
only,  instead  of  a  plunger,  a  heavy  piston  is  employed. 

The  object  of  the  valve  in  the  discharge-pipe,  H,  is  to  relieve  the 
lower  valve  from  strain,  while  the  piston  is  descending. 

185.  Figure  30.— Double-acting  suction  and  force  pump 
with  two  valves. — This  pump,  though  it  throws  a  steady  stream 

without  an  air-chamber,  has  but  two 
valves;  but  it  has  two  cylinders  and  two 
pistons;  and  is  constructed  as  shown  in 
the  figure. 

OPERATION. — By  means  of  the  cross- 
bar, A,  both  pistons  are  simultaneously 
moved  in  the  same  direction.  As  they 
are  raised  up,  the  upper  valve  in  piston 
N  closes,  and  the  water  in  it  is  discharged 
through  the  pipe,  H,  and  the  vacuum  pro- 
duced under  both  pistons  is  filled  through 
the  lower  valve,  T.  By  reversing  the  mo- 
tion, not  only  will  the  water  in  the  left- 
hand  cylinder  pass  through  the  upper 
valve,  but  the  water  below  the  piston,  L, 
will  also  be  forced  through  it ;  and  thus  a 
quantity  of  the  liquid,  equal  to  the  ca- 
pacity of  the  right-hand  cylinder,  will  be 
forced  into  the  discharge-pipe,  as  shown 
by  the  long  arrow,  while  the  piston  is  des- 
cending, which,  of  course,  keeps  up  a  con- 
stant discharge. 

186.  Figure  31. — Fire-engine.— This  important  machine  is  a 
four-valve,  two-cylinder,  double-acting  suction  and  force  purnp,  pro- 
vided with  a  strong  air-chamber.     Connected  with  the  piston-rods  is  a 
powerful  lever,  in  the  extremities  of  which,  VY,  are  provided  long  bars 
or  rods  of  wood  (not  shown),  by  means  of  which  many  men  can  work 
the  engine  at  the  same  time. 

OPERATION. — As  one  piston  descends  the  other  ascends.  Supposing 
the  piston,  F,  to  be  forced  down,  the  valve,  E,  will  close,  and  the  water 
in  this  cylinder  will  be  driven  through  the  valve,  H,  into  the  air- 
chamber  ;  the  valve,  L,  will  be  closed  to  prevent  the  water  from  escap- 
ing into  the  cistern,  and  the  valve,  A,  will  be  opened  by  the  water  from 


HYDRAULICS. 


133 


the  supply-pipe,  as  indicated  by  the  long  arrows.     Reversing  the  mo- 
tion will  reverse  the  valves.     From  the  air-chamber  the  water,  W,  is 

FIG.  31. 


forced  through  the  pipe,  T.  The  object  of  the  air-chamber  is  to  pro- 
duce, by  means  of  the  elasticity  of  the  compressed  air,  a  steady  and 
continuous  jet  from  the  nozzle  of  the  hose-pipe. 

Fire-engines  were  first  employed  in  Egypt.  Not  only  were  such 
engines  used  in  early  times,  but  the  construction  of  those  of  remote 
antiquity  was  similar  to  that  of  the  ordinary  machines  now  employed ; 
including  even  the  air-chamber,  notwithstanding  it  is  supposed  the 
ancients  were  wholly  unacquainted  with  atmospheric  pressure. 

Rotary  pumps,  worked  by  steam,  are  rapidly  taking  the  place  of  these 
hand-engines,  one  of  which  is  more  effective  than  a  dozen  of  the 
ordinary  rmichincs. 


134 


HYDRA  ULICS. 


FIG.  32. 


187.  Figure   32.— Stomach  pump. — The  object  of  this  use- 
ful instrument  is  to  throw  fluid  into  the  stomach,  and  then  withdraw 

it,  without  changing  the  apparatus,  only 
by  altering  its  position.  It  consists  of  a 
syringe,  A,  provided  with  two  ball-valves, 
T  and  H,  and  two  flexible  pipes,  D  and 
S,  to  be  the  required  length. 

OPERATION.  —  The  syringe  must  be 
held  horizontally,  and  the  pipe  that  is  to 
bring  the  fluid  to  the  syringe  must  have 
its  valve  lowermost.  For  instance,  if  the 
pipe,  S,  comes  from  the  stomach,  then, 
by  drawing  out  the  handle  of  the  syringe, 
the  fluid  coming  from  the  stomach  would 
lift  the  ball,  H;  by  reversing  the  motion 
of  the  piston,  this  ball  would  fall,  and 
the  other  ball,  T,  be  raised  by  the  con- 
tents of  the  syringe,  as  indicated  by  the 
arrow.  Then,  to  reverse  the  flow  of  the 
fluid,  to  pump  liquids  into  the  stomach, 
without  withdrawing  the  pipe,  it  is  only 
necessary  to  place  the  pipe,  D,  in  the 
fluid,  and  turn  the  valve,  T,  lower- 
most. 


Siphons,  Fountains,  etc. 

188.  Figure  33.— The  siphon  dependent  on  atmospher- 
ic pressure. — The  siphon  is  used  principally  for  decanting  liquids, 
and  consists  of  a  bent  tu'be,  having  one  of  its  arms  longer  than  the 
other  ;  and  depends,  for  its  operation,  upon  atmospheric  pressure.  To 
put  it  in  operation,  fill  it  with  fluid  and  put  the  mouth  of  the  short 
arm  in  to  the  liquid. 

OPERATION". — Suppose  the  long  dotted  arrow  in  the  siphon,  HL 
(Fig.  34),  to  be  a  chain  passing  over  a  pulley  at  L,  and  it  is  evident 
that  the  greater  weight  of  the  longer  end  would  cause  it  to  fall  out  of 
the  pipe.  So,  if  the  siphon  were  filled  with  water,  the  fluid  in  the 
longer  arm  being  the  heavier,  it  would  commence  to  fall,  and  if  the 
mouth  of  the  short  arm  were  in  water,  it  would,  by  atmospheric  pres- 
sure, supply  the  long  arm ;  and  so  it  would  continue  to  flow. 

The  velocity  of  the  flow  will  be  the  same  as  if  the  liquid  fell  freely 
from  a  height  equal  to  the  distance  between  the  level  of  the  liquid  in 
the  vessel  and  the  end  of  the  long  arm ;  or,  from  L  to  H  in  Fig.  33. 


HYDRAULICS. 


135 


To  prove  that  this  instrument  operates  by  pressure  of  air,  place  the 
short  arm  in  an  air-tight  vessel,  N  (Fig.  33),  provided  with  a  faucet 
When  the  siphon  is  operating,  close  the  faucet  and  the  flow  will  cease; 
open  the  faucet  and  the  flow  will  again  commence. 

As  siphons  operate  by  pressure  of  the  air,  the  short  arm  cannot  be 
over  34  feet  long;  ttiat  is,  its  vertical  height  must  not  exceed  this  dis- 
tance ;  though  the  short  arm  may  be  many  times  this  length,  and  the 
long  arm  but  a  small  part  of  34  feet;  it  being  required,  only,  that  the 
discharging  arm  have  a  vertical  height  greater  than  that  of  the  receiv- 
ing arm,  irrespective,  too,  of  their  relative  size. 

Very  short  siphons  will  work  in  a  vacuum,  by  the  tenacity  of  the 
liquid,  caused  by  the  force  of  cohesion  among  its  particles  (36). 


FIG.  33. 


FIG.  34. 


189.  Intermittent  springs.— There  are  in  nature  intermittent 
springs,  the  water  flowing  regularly  for  a  time,,  and  then  suddenly  ceasing. 
One  of  these  is  illustrated  by  Fig.  7,  Chart  2  (99),  in  which  T  is  a  sub- 
terranean cavity,  having  an  outlet,  L,  shaped  like  a  siphon.  As  the 
water  now  stands,  the  opening  of  the  short  arm  is  under  water,  but 
when  the  water  falls,  so  air  can  enter  it,  the  spring  will  instantly  cease, 


13<>  HYDRAULICS. 

and  not  flow  again  until  the  water  from  the  hills,  HH,  above,  fills  the 
cavity  up  to  the  level  of  the  highest  point  of  the  siphon,  L  (shown  by 
the  dotted  line),  which  may  require  a  long  time. 

190.  Figure  34.— Sharp  angles  obstruct  the  flow  of 
liquids,  shown  by  siphons. — Let  the  two  siphons  in  this  figure 
be  every  way  alike,  except  one  has  an  oval  bend,  and  the  other  a  sharp 
turn ;  and  it  is  found  that  the  one  with  a  rounded  turn  will  discharge 
much  more  rapidly  than  the  other. 

FIG.  35. 


191.  Figure  35.— Conveying  water  over  hills  with  si- 
phons.— The  ancients  made  extensive  use  of  the  siphon  for  conveying 
water  over  elevations.  Let  the  diagram  represent  a  hill,  which  may  ex- 
tend many  miles  over  ;  and  if  its  vertical  height  be  not  more  than  about 
33  feet  from  the  level  of  the  supply- water — and  the  supply-water  not  far 
above  the  level  of  the  sea — the  liquid  can  be  conveyed  over  it  with  a  pipe, 
as  shown  by  the  dotted  line.  To  put  such  a  siphon  as  this  in  opera- 
tion, first  plug  up  both  ends  of  the  pipe,  and  then  fill  it  by  means  of  an 
opening  at  its  highest  point,  then  close  the  opening  and  draw  the  plugs. 
The  diagram  will  be  otherwise  understood  without  further  description. 


HYDRA  ULIClS. 


137 


2.  Figure  36.— Siphon  for  the  chemical  laboratory. 

—As  it  is  often  necessary  to  employ  the  siphon  in  acids  and  other 
liquids  which  are  unpleasant  to  handle,  they  are  so  constructed  that 
they  can  be  charged  by  sucking.  Such  an  one  is  illustrated  in  the  fig- 
ure. The  mouth  is  placed  at  N,  and  the  finger  at  the  bottom  of  the 
open  tube  below.  When  the  liquid  reaches  the  bulb,  N,  withdraw  the 
mouth,  then  remove  the  finger,  and  the  siphon  will  operate.  S  shows 
the  sediments  in  the  jar,  which,  by  means  of  this  instrument,  are  left 
undisturbed,  while  the  clear  fluid  is  being  drawn  off. 

FIG.  36.  FlG.  37> 


1 93.  Figure  37. — Loss  of  effective  head  in  public  water- 
works.— Though  the  head  in  the  reservoir  of  public  water-works 
may  remain  the  same  at  all  times,  yet,  at  remote  points,  the  water,  es- 
pecially during  business  hours,  will  not  rise  to  the  same,  or  even  to  an 
approximate,  level.  This  is  because  the  pressure,  at  any  given  place,  is 
more  or  less  diminished  by  every  other  opening  intervening  between 
this  place  and  the  reservoir;  which  is  in  accordance  with  the  law  re- 
lating to  pressure  of  confined  fluids — that  pressure  increased  or  dimin- 
ished at  one  point,  is  increased  or  diminished  at  every  other  point. 


!38  HYDRAULICS. 

Suppose  A,  in  the  figure,  to  represent  the  reservoir,  communicating 
with  the  horizontal  section  at  the  bottom,  in  which  there  are  six  open- 
ings, as  shown,  two  of  them  being  small  jets.  These  jets  do  not  rise 
to  only  about  half  the  height  of  the  level  of  the  water;  but  if  one  of 
the  other  apertures  be  closed,  they  will  rise  higher ;  and  if  they  were 
all  closed,  the  jets  would  rise  nearly  to  the  level  of  the  reservoir. 

19^.  Lateral  pressure  of  liquids  diminished  by  motion. 

—The  lateral  pressure  of  water  in  pipes  is  also  diminished  by  the 
velocity  of  the  fluid  within  them;  that  is.  if  a  jet  issue  from  the  side 
of  a  large  pipe  of  running  water,  under  a  given  head,  it  will  not  rise  so 
high  as  when  there  is  no  current  within  the  main  pipe. 

Fountains,  etc. 

195.  Fountains  and  vertical  jets  of  water.— According  to 
FIG.  38.  the  laws  of  falling  and  rising  bodies 

(55)  and  pressure  of  liquids,  vertical 
jets  should  rise  to  the  level  of  the  water 
in  the  reservoir,  but  this  never  quite 
takes  place,  because  of,  1st,  the  fricti  >n 
in  the  tubes  diminishing  the  velocity ; 
2d,  the  resistance  of  the  air ;  3d,  the 
returning  water  falling  upon  that 
which  is  rising. 

196.  Figure  38.  --  Hiero's 
fountain. — In  this  curious  fountain 
the  water  seems  to  rise  higher  than  its 
source,  by  one  body  of  water  acting 
upon  another  through  an  intervening 
column  of  air.  There  are  many  mod- 
ifications in  its  form,  the  one  repre- 
sented illustrating  the  principle  rather 
than  the  deception. 

This  fountain  may  be  considered  the 
oldest  pressure-engine  known,  a  vol- 
ume of  air  acting  as  the  piston. 

OPERATION.— Construct  the  appara- 
tus as  shown.  Through  the  nozzle, 
fill  the  bulb,  L,  nearly  full  of  water, 
and  close  the  faucet.  Then  pour  water 
into  the  vessel,  H,  until  it  is  nearly 
filled,  as  shown.  The  water  in  II  is 


HYDRAULIC. 


139 


prevented  from  passing  through  the  bulb,  T,  and  so  up  into  the  bulb 
L,  by  the  compressed  air  intervening  between  the  water  in  T  and  in  L. 
The  water  in  L,  through  the  medium  of  the  compressed  air,  is  now 
pressed  with  a  force  equal  to  a  column  of  water  extending  from  its 
surface  in  T,  to  its  surface  in  H.  Hence,  if  the  faucet  be  opened,  the 
water  from  the  bulb,  L,  will  rise  to  a  distance  above  its  surface  equal 
to  the  height  of  the  column  from  H  to  T.  It  will  be  observed  that 
none  of  the  liquid  from  H  to  T  passes  out  of  the  nozzle ;  this  water 
serving  only  to  supply  the  force  which  drives  the  water  out  of  the 
bulb,  L. 

By  ingeniously  combining  the  two  long  pipes  and  two  upper  vessels, 
H  and  L,  so  that  one  pipe  and  vessel  will  contain  and  conceal  the 
other,  a  person  unacquainted  with  the  apparatus  could  easily  believe 
that  water,  after  all,  will  rise  higher  than  its  source. 


197'  Figure  39.— Intermittent  fountains. — An  intermit- 
tent fountain  is  one  in  which  the  flow  takes  place  at  regular  intervals. 
These  exist  in  nature.  An  artificial 
one  is  represented  in  the  figure. 

Fasten  a  glass  globe,  closed  with 
a  stopper,  on  the  upright  tube,  N, 
so  that  the  tube  will  extend  quite 
to  the  top  of  the  vessel ;  provide 
two  small  pipes  at  the  bottom  of  the 
globe  for  jets,  and  secure  the  lower 
end  of  the  tube  to  the  bottom  of  the 
basin,  H.  Around  the  bottom  of 
the  tube  provide  small  holes  through 
which  air  can  enter  it,  and  thus 
reach  the  upper  part  of  the  globe. 
The  two  side  apertures  in  the  basin 
must  be  smaller  than  the  jets. 

OPERATION. — The  globe  and  ba- 
sin are  first  nearly  filled  with  water. 
The  small  holes  at  the  foot  of  the 
tube,  N,  being  above  the  water,  will 
allow  the  air  to  pass  above  the  sur- 
face of  the  liquid  in  the  globe/ and 
the  water  will  flow  from  the  jets 
into  the  basin  until  it  is  filled  so  as 
to  cover  these  small  holes,  which 
shut  off  the  air  from  the  globe,  thereby  causing  the  flow  of  the  foun- 


140  HYDRAULICS. 

tain  to  cease.  But  when  the  water  has  been  drawn  from  the  basin, 
by  its  apertures,  so  as  to  again  expose  the  small  holes  to  the  air,  the 
fountain  will  again  begin  to  flow,  and  so  on. 

Importance  of  water. — Water,  in  many  respects,  is  the  most 
important  substance  known.  Every  comfort  of  civilized  or  savage  life 
is  more  or  less  dependent  upon  it.  Without  it,  vegetation  would  cease, 
and  every  animated  being  would  perish.  It  enters  into  nearly  every 
combination  of  matter.  Even  the  atmosphere  we  breathe,  deprived  of 
moisture,  would  destroy  life.  The  mechanical  effects  produced  by  it 
render  it  of  equal  importance  in  the  arts.  It  was  the  earliest  source 
of  inanimate  motive-power,  and  without  it,  man  could  not  bring  to  his 
aid  that  mighty  machine,  the  steam-engine. 

Importance  of  hydraulic  and  hydro-pneumatic  ma- 
chines.— The  universal  importance  of  water,  and  its  scarcity  in  many 
countries,  and  the  fact  that,  in  its  liquid  form,  it  always  seeks  the  lowest 
levels  and  places  of  the  earth,  while  it  is  ever  needed  at  more  elevated 
situations,  and  often  in  vast  quantities,  as  for  irrigating  lands  and 
supplying  large  cities,  render  water-elevators  of  the  utmost  importance. 
Hence,  from  the  earliest  times,  the  ingenuity  of  man  has  been  at  work 
to  produce  machines  for  this  purpose ;  which  finally  led  to  the  inven- 
tion of  the  steam-engine,  which  now  not  only  excels  all  other  means 
for  this  purpose,  but,  more  than  all  other  mechanical  devices,  is  sub- 
serving the  general  wants  of  civilization. 


HEAT.  141 


CHAPTER    IX. 

(CHART  NO.  4.) 

HEAT   AND    STEAM-ENGINE. 
Preliminary  General  Principles  relating  to  Heat. 

198.  Definitions. — The  term  caloric  is  the  agent  which  excites 
in  our  bodies  the  sensation  of  heat.    For  convenience,  however,  in  this 
work  these  terms  are  used  synonymously. 

199.  Heat  and  cold  relative  terms.—  Heat  and  cold  are  only 
relative  terms,  cold  being  the  absence  of  heat.      That  is,  any  body, 
whatever  its  temperature,  is  said  to  be  hot  when  compared  with  bodies 
colder  than  itself,  and  cold  when  compared  with  bodies  hotter  than 
itself.     Hence,  we  say,  ordinarily,  that  bodies  are  warm  or  hot  and 
cool  or  cold  when  they  seem  so  to  the  touch,  whatever  be  the  tempera- 
ture of  the  hand  at  the  time. 

200.  Temperature. — The  term  temperature  does  not  imply  the 
quantity  of  heat  in  a  body,  but  merely  its  relative  heat,  at  a  given 
time,  as  compared  to  an  arbitrary  standard — like  the  common  ther- 
mometer. 

201.  Nature  of  heat. — The  real  nature  of  heat  is  unknown. 
Scientific  opinion  is  divided  between  two  views  respecting  it.     These 
are  the  corpuscular  or  emission  theory  and  the  undulatory  theory. 

According  to  the  emission  theory,  heat  is  a  subtile  fluid,  destitute  of 
weight,  existing  in  all  bodies,  is  capable  of  flowing  from  one  body  to 
another,  and,  though  attracted  by  particles  of  other  bodies,  its  own 
particles  mutually  repel  each  other. 

According  to  the  undulatory  theory,  heat  is  attributed  to  the  vibra- 
tory movements  of  the  molecules  of  a  hot  body  communicated  to  those 
of  other  bodies  by  means  of  a  highly  elastic  fluid  called  ether,  in  the 
same  manner  that  sound  is  transmitted  through  air. 

This  ether  pervades  all  space,  and,  by  other  kinds  of  motions,  is 
supposed  to  produce  light  and  electricity. 


142  HEAT  AND  STEAM-ENGINE. 

By  the  former  theory,  bodies  cool  by  losing  a  portion  of  this  subtile 
fluid ;  by  the  latter,  they  simply  lose  a  part  of  their  vibratory  motion. 

Herein  the  emission  theory,  for  convenience  of  explanation,  is  as- 
sumed, though  at  present  the  undulatory  theory  is  generally  received. 


.  General  effects  of  heat. — 1st.  Heat,  by  penetration,  unites 
with  the  ultimate  molecules  (4  and  5)  of  all  bodies,  and,  with  its  repel- 
lent forces,  counteracts  those  of  cohesion  (22  and  23),  and  thus  ex- 
pands all  bodies.  Particles  of  todies,  with  sufficient  heat,  are  so  far 
repelled  as  to  move  freely  among  themselves,  and  so  become  liquids; 
•and  with  still  greater  heat,  the  liquids  pass  into  a  state  of  vapor.  Va- 
pors, if  deprived  of  heat,  return  to  the  liquid  state  ;  and  the  liquids,  by 
further  abstraction  of  heat,  become  solids  (88) ;  and,  if  the  process  be 
continued,  the  solids  go  on  contracting.  Hence,  heat  dilates,  and  cold 
contracts,  bodies,  but  in  different  degrees.  The  most  dilatable  are 
gases,  then  vapors,  then  liquids,  and  finally  solids.  Hence,  heat  deter- 
mines the  size  and  state  of  bodies. 

2d.  Heat,  by  its  powerful  repellent  force,  so  readily  and  extensively 
expands  other  bodies,  that  it  has  come  to  be  the  most  available  and 
universally-employed  inanimate  power  known.  In  its  application  to 
the  expansion  of  water  to  steam,  it  is  already  exerting  more  mechanical 
energy  in  subjugating  the  earth,  and  otherwise  subserving  the  wants 
of  man,  than  all  the  inanimate  forces  of  the  globe  combined. 

3d.  The  distribution  of  heat  on  the  earth's  surface  determines,  prin- 
cipally, the  distribution  of  animals  and  plants. 

4th.  This  agent  or  fluid  has  very  great  control  over  all  chemical 
transformations  of  substances.  In  some,  heat  is  evolved  ;  in  others, 
cold  is  produced. 

5th.  The  power  of  heat  is  not  limited  to  the  inorganic  world.  Life, 
on  this  planet,  cannot  take  place  only  within  certain  limits  of  temper- 
ature, that  is,  between  the  freezing  and  boiling  points  of  water;  while 
variations  of  its  intensity,  within  these  limits,  are  indispensable  to  the 
laws  of  vitality  and  physiological  changes. 

203.  Equilibrium  of  heat. — A  heated  body  (whatever  its 
temperature)  surrounded  by  cooler  bodies,  gives  off  its  heat,  and  the 
surrounding  bodies  attract  and  take  it  up.  This  process  will  go  on. 
until  it  and  all  the  adjacent  bodies  have  a  common  temperature  ;  or  if 
a  cold  body  be  surrounded  by  hotter  on,es,  then  it  will  attract  heat 
from  all  the  rest,  which  give  it  off,  until  the  intensity  is  uniform  in 
all.  This  removal  is  termed  transference  ;  and  when  the  temperature 
has  declined  or  increased  to  that  of  the  adjacent  bodies,  an  equilibrium 
is  said  to  have  been  attained. 


HEAT.  143 

There  are  two  methods  by  which  this  transference  takes  place :  1st, 
by  radiation— both  general  and  interstitial ;  2d,  by  convection. 

204-  Luminous  and  obscure  heat. — Heat  radiated  from  non- 
luminous  bodies,  as  a  ball  heated  below  redness,  is  called  obscure  heat ; 
that  radiated  from  luminous  bodies,  as  the  sun,  or  a  ball  heated  to 
redness,  is  called  luminous  heat. 

SOURCES     OF     HEAT. 

There  are  four  principal  sources  of  heat:  physical, chemical,  mechan- 
ical, and  physiological. 

Physical  Sources  of  Heat. 

Physical  sources  of  heat  are  solar  radiation,  stellar  radiation,  terres- 
trial radiation,  and  electricity. 

205.  Solar   radiation  is  the  principal  source  of  heat  to  our 
globe ;  though  the  distance  of  the  sun  from  us  is  95,000,000  of  miles. 
Its  size  or  volume  is  1,400,000  times  greater  than  the  earth.     Opinions 
are  divided  as  to  the  cause  of  the  sun's  heat. 

206.  Quantity  of  heat  emitted  by  the  sun.— The  quantity 
of  heat  annually  received  by  the  earth  from  the  sun,  is  sufficient  to 
melt  a  crust  of  ice  surrounding  the  earth  101  feet  thick.     The  atmo- 
sphere absorbs  nearly  40  per  cent,  of  this  heat.     It  is  also  estimated 
that  the  whole  amount  of  heat  emitted  by  the  sun  is  2,381,000,000  times 
greater  than  that  received  by  the  earth ;  which  is  sufficient  to  rnelt  a 
cylinder  of  ice  45  miles  thick,  at  the  rate  of  190,000  miles  a  second. 

20  7.  Extremes  of  natural  temperature,  on  the  earth,  vary 
from  70°  F.,  below,  to  146°  above,  zero.  In  this  latitude  (New  York), 
from  the  coldest  to  the  hottest  seasons,  the  variations  are  often  110°,  F. 

208.  Terrestrial  radiation. — There  are  many  theories  regard- 
ing terrestrial  heat,  but  it  may  be  partly  accounted  for  by  local  chem- 
ical action. 

The  heat  of  the  sun  does  not  penetrate  the  earth  more  than  from  50 
to  100  feet.  Descending  into  the  earth,  after  passing  the  point  of  con- 
stant temperature,  the  heat  regularly  increases  about  1°.8  for  every  100 
feet.  At  the  depth  of  2  miles,  water  would  boil ;  at  23  miles,  cast  iron 
would  melt.  Hence,  it  is  estimated  that  the  crust  of  the  earth  is  not 
more  than  100  miles  thick  ;  or  one-fortieth  of  its  radius;  which,  re- 
latively, is  thinner  than  an  egg-shell. 


144  HEAT  AND  STEAM-ENGINE. 

209.  Atmospheric  electricity  is  a  source  of  heat  which  is  but 
little  understood.     Its  intense  calorific  powers  are  shown  by  flashes  of 
lightning   not   unfrequently  fusing   metals    and   earthy   matter,  and 
cutting  chains  and  bars  of  iron  in  two,  as  with  a  blade  of  fire,  instan- 
taneously. 

Chemical  Sources  of  Heat. 

210.  Chemical  sources  of  heat.  —  When  chemical  combination 
of  two  substances  takes  place,  it  is  usually  attended  with  an  elevation 
of  temperature,  but  sometimes  with  a  depression. 


Combustion.  —  When  the  heat  developed  by  the  chemical 
combination  of  two  bodies  produces  luminosity,  the  bodies  are  said  to 
burn,  and  the  phenomenon  is  termed  combustion.  If  one  of  the  bodies 
be  a  solid,  it  is  called  fire  ;  if  gaseous,  flame.  Oxygen  combining  with 
other  substances,  constitutes  all  ordinary  combustion.  It  is  supposed 
that  the  cause  of  heat  in  combustion  is  the  vibratory  motions  of  the 
constituent  atoms  of  the  bodies,  as  they  combine  together.  The  amount 
of  heat  evolved  by  chemical  combination  varies  with  different  sub- 
stances. 


.  Mechanical  sources  of  heat.  —  The  principal  of  these 
are  friction,  compression,  and  percussion. 

When  two  bodies  are  rubbed  together,  heat  is  generated  by  the 
friction  of  their  surfaces;  which  must  be  attributed  to  a  molecular 
movement  of  the  bodies,  excited  by  the  friction. 

The  quantity  of  heat  developed  by  friction  depends,  1st,  on  the  na- 
ture and  state  of  the  surfaces  ;  2d,  on  the  pressure  ;  3d,  on  the  velocity. 

Compression.  —  When  any  substance  is  diminished  in  volume,  there 
is,  generally,  a  development  of  heat;  strikingly  seen  in  gases,  when 
they  are  suddenly  compressed  (236). 

Percussion  is  a  combination  of  friction  and  compression,  produced 
by  hammering  ;  and  the  heat  evolved  is  principally  due  to  the  diminu- 
tion of  bulk  of  the  body  hammered. 

213.  Physiological  source  of  heat.  —  Animal  heat  is  the 
result  of  a  series  of  chemical  actions  taking  place  within  the  living 
body,  the  principal  of  which  is  combustion  in  the  lungs  ;  oxygen  of 
the  air  combining  with  the  carbon  and  hydrogen  of  the  blood,  forming 
carbonic  acid  and  vapor  of  water.  The  heat  thus  developed  is  equal 
to,  and  so  compensates  for,  that  lost  from  the  exterior,  which  keeps  the 
body  at  a  uniform  temperature  in  all  climates  and  seasons. 


HEAT.  Ho 

Vegetable  life  is  also  attended  with  chemical  changes  and  consequent 
evolution  of  heat.  The  temperature  of  plants  is,  in  general,  from  0°.9 
to  l°.l  higher  than  that  of  the  surrounding  air. 


Difference  between  quantity  and  intensity  of 
heat.  —  No  amount  of  heat  of  low  temperature  can  be  so  applied  to 
another  object  as  to  raise  it  to  a  higher  temperature  than  that  of  the 
source  from  which  the  heat  emanated.  For  instance,  with  a  lens  the 
heat  of  the  sun  will  ignite  combustible  substances,  but  the  same  quan- 
tity of  solar  heat,  after  being  absorbed  by  a  blackened  wall  and  then 
radiated,  cannot  be  brought  to  that  degree  of  intensity  necessary  to 
ignite  the  same  substances.  For  the  same  reason  solar  heat,  reflected 
from  the  moon,  loses  its  intensity. 

EXPANSION. 

Linear  Expansion. 

215.  Figure  1.—  Linear  expansion  of  solids.—  Pyrome- 
ters. —  Place,  any  kind  of  metallic  rod  in  the  two  supports,  L  and  H, 

FIG.  l. 


so  that  one  extremity  will  abut  against  the  short  arm  of  the  lever,  A, 
when  it  stands  in  a  vertical  position ;  then  secure  the  other  extremity 
by  the  clamp-screw  H.  When  the  rod  is  heated,  by  lighting  the  lamps, 
it  will  expand ;  and  the  extent  of  expansion  will  be  indicated  by  the 
movement  of  the  long  arm  of  the  lever  on  the  graduated  index.  If  the 
lamps  be  extinguished,  the  bar,  on  cooling,  will  contract  to  its  original 
length ;  shown  by  the  lever  again  returning  to  the  vertical  position. 

10 


J  4( !  HE  A  T  AND  STEAM-ENGINE. 

This  instrument  is  called  the  pyrometer  (there  are  other  and  better 
ones),  and  by  its  use  it  is  found  that  the  laws  of  expansion  are : 

1.  The  extent  of  the  expansion  and  contraction  of  the  same  metal 
varies  with  variations  of  temperature. 

2.  The  extent  of  the  expansion  and  contraction  of  different  metals 
varies  with  the  same  temperature. 

Iron  rail-tracks  are  laid  with  reference  to  the  first  law,  by  leaving  a 
space  between  the  ends  of  the  rails,  to  prevent  the  track  from  " lifting" 
as  it  is  called. 

The  second  law  is  practically  applied  in  the  construction  of  compen- 
sating pendulums  (60). 

The  contraction  and  expansion  of  metals  by  heat  and  cold  are  also 
employed  to  exert  powerful  force  through  short  distances ;  as  setting 
tires  on  wheels,  drawing  walls  of  buildings  together,  etc. 

The  immense  Croton  water-pipe  at  High-Bridge,  New  York,  rests 
on  rollers  to  facilitate  its  movement,  caused  by  expansion  and  con- 
traction. 

The  Niagara  Suspension  Bridge  is  deflected  only  about  six  inches  by 
the  heaviest  trains  of  cars,  while  the  expansion  and  contraction  of  its 
cables,  by  change  of  temperature,  cause  the  bridge  to  rise  and  fall  about 
two  feet. 

The  cables  of  the  suspension  bridge  being  constructed  between  New 
York  and  Brooklyn,  will  expand  and  contract  about  seven  feet;  to 
provide  for  which,  the  towers  over  which  the  cables  will  pass  are  to  be 
160  feet  higher  than  the  floor  of  the  bridge. 

2. 216.  Co-efficient  of  expansion. 

— The  co-efficient  of  lineal  expansion  is 
the  small  amount  gained  in  the  length 
of  a  rod,  a  foot  long,  when  heated  from 
32°  to  33°  F. 

The  co-efficient  of  cubic  expansion  is 
the  small  fraction  of  its  volume,  by 
which  a  solid,  liquid,  or  gas  is  increased 
when  heated  from  32°  to  33°  F. 

Cubic  Expansion. 

217.  Figure  2. —  Cubic  Ex- 
pansion of  solids. — By  this  is  meant 
expansion  in  three  directions,  or  ex- 
pansion of  volume.  T  is  a  metallic  ball 
of  just  sufficient  magnitude,  at  ordinary 
temperature,  to  fit  between  the  upright 


HEAT. 


147 


jirms  of  the  bed-piece,  M,  as  shown.  If  the  ball  be  taken  out  and 
heated  in  a  furnace,  it  cannot  be  placed  between  the  arms,  in  whatever 
position  it  be  turned  ;  showing  it  has  been  expanded  in  all  directions. 
If  it  be  left  on  the  points  of  the  arms,  as  shown,  it  will,  on  cooling,  drop 
into  its  original  position. 

If  the  experiment  be  performed  with  a  cube,  instead  of  a  sphere,  the 
result  will  be  the  same. 

Different  solids  expand  unequally  with  the  same  heat  J  and  the  same 
xolids  unequally  with  different  degrees  of  heat. 


Relation  between  linear  and  cubical  expansion.— 

If  the  linear  expansion  is  owe-  thousandth  of  the  original  length,  then 
the  cubical  expansion  will  be  three  one-thousandths  of  the  original 
bulk. 

219.  Amount  of  expansion  of  solids,  absolute  and  rela- 
tive. —  The  most  expansible  metal,  zinc,  increases  only  one  three 
hundred  and  fortieth  (-j£¥)  of  its  length,  with  change  of  temperature 
from  the  freezing  to  the  boiling  point  :  while  glass  expands  only  one- 
third  as  much  as  zinc.  The  relative  expansibility  of  metals  and  glass 
is  as  follows,  commencing  with  the  most  and  ending  with  the  least 
expansible:  zinc,  lead,  tin,  silver,  brass,  gold,  copper,  bismuth,  iron, 
steel,  antimony,  platinum,  glass. 

The  compressibility  of  these  substances  is  about  in  the  same  order. 
The  most  expansible  are,  in  general,  the  most  fusible. 

The  ratio  of  expansion  increases  with  the  temperature. 


EXPANSION    OP    SOLIDS. 

By  increasing  the  temperature  from  32°  to  212°  F. 


SUBSTANCES. 

EXPANSION. 

IN 

LENGTH. 

IN 

BULK. 

English  Flint-glass     

1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 

in 

a 

« 
a 

u 
it 
it 
(4 
(t 
(t 
It 

1248 
1131 

926 
847 
682 
582 
536 
524 
516 
351 
340 

1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 

in  316 

"  377 
«  309 
"  282 
"  227 
"  194 
«  179 
"  175 
«  172 
"  117 
"  113 

Platinum  

Tempered  Steel            

Iron  

Gold  

Copper  

Brass  

Silver  

Tin                     

Lead        

Zinc                           

[48  HEAT  AND  STEAM-ENGINE. 

Expansion  of  Liquids. 
.  Figure  3.  —  Expansion  of  liquids.—  All  liquids  expand 


by  heat  more  than  solids;  for,  irfercury,  the  least  expansible  of  all 
liquids,  expands  more  than  the  most  expansible  metal,  which  is  zinc. 
Their  expansibility,  however,  is  more  variable  than  that  of  liquids. 

FIG.  3. 


Fill  the  bulb  of  the  glass  vessel,  A,  in  the  figure,  up  to  the  tube, 
with  any  fluid,  then  place  the  bulb  over  a  spirit-lamp,  and  as  the  liquid 
becomes  heated,  its  expansion  will  be  shown  by  its  rising  in  the  tube  ; 
and,  on  being  allowed  to  cool,  it  will  return  to  its  original  bulk. 


.  The  amount  of  expansion  of  liquids.—  Liquids  expand 
unequally  for  equal  increments  of  heat. 

The  same  liquid  expands  more  the  more  it  is  heated;  and  the  higher 
the  temperature  rises  the  greater  will  be  the  expansion  for  a  given 
increase  of  heat.  This  is  owing  to  the  fact  that  the  higher  the  tem- 
perature becomes,  the  further  the  particles  are  removed  from  each 
other,  and,  therefore,  the  cohesive  or  antagonistic  power  (23)  dimin- 
ishes in  a  greater  ratio  than  the  repulsive  force  of  heat  increases. 


.  Different  liquids  expand  differently  for  the  same 
increase  of  temperature.—  In  the  diagram  (Fig.  3)  let  A,  W,  and 
M  be  three  glass  bulbs  of  equal  dimensions,  with  narrow  graduated  tubes, 
filled,  each  to  the  same  level,  with  different  liquids;  say,  alcohol,  water, 
and  mercury.  On  pouring  hot  water  into  the  trough,  the  fluids  will 


HEAT.  149 

expand  and  rise  in  the  tubes ;  but,  not  expanding  equally,  they  do  not 
rise  to  the  same  height,  as  indicated  by  the  arrows.  The  alcohol  in 
A  will  rise  the  highest,  and  the  mercury  in  M  the  least ;  while  the 
water  in  W  will  rise  to  a  point  intermediate  between  the  other  two. 


EXPANSION     OP     LIQUIDS, 

By  increasing  the  temperature  from  32°  to  212°  F. 

Mercury  ..........................  1  part  in  55. 

Pure  Water  .......................  1     "     «   21.3 

Sulphuric  Acid  ....................  1     "     "   17. 

Oil  of  Turpentine  ..................  1     «     "14. 

Fixed  Oils  ........................  1     "    «   12.5 

Alcohol  ...........................  1     "    «     9. 

The  most  expansible  liquids,  generally,  are  those  whose  boiling 
points  are  the  lowest. 

223.  Water,  at  certain  temperatures,  an  exception  to 
the  laws  of  contraction  and  expansion.  —  Water,  in  cooling, 
ceases  to  contract  at  about  42°  F.  ;  and  at  about  39°,  just  before  it 
reaches  the  freezing  point  (32°),  it  begins  to  expand  again,  and  more 
and  more  rapidly  as  the  freezing  point  is  reached.  This  expansion  is 
about  one-eleventh  of  its  bulk,  and  accounts  for  the  bursting  of  pipes, 
vessels,  etc.,  when  water  is  freezing  within  them. 


-  Beneficial  effects  of  unequal  expansion  of  water. 

—  By  this  exception  to  the  law  of  expansion,  ice  is  rendered  specifically 
lighter  than  water,  which  causes  it  to  float,  and  so  form  a  cover  to 
lakes,  rivers,  etc.  ;  and,  being  a  good  non-conductor  of  heat,  it  prevents 
radiation  from  the  water  below  it  ;  and  thus  keeping  the  water  warm, 
preserves  animal  life  below,  and  prevents  the  accumulation  of  vast 
quantities  of  ice.  If  it  were  not  for  this  grand  exception  to  the  other- 
wise general  law,  ice  would  sink  and  fill  the  beds  of  rivers,  causing 
them  to  overflow,  and  be  exposed  in  large  surfaces  to  the  air;  and  so, 
besides  inundating  the  country,  produce  fields  and  masses  of  ice  that, 
in  higher  latitudes,  would  not  melt  from  one  cold  season  to  another; 
and  thus  render  even  our  own  climate  uninhabitable. 

22  5.  Freezing  of  water  in  small  tubes.—  Water  freezes  at 
a  much  lower  temperature  than  32°  in  small  tubes,  like  sensible  pores  ; 
which,  doubtless,  is  one  of  the  many  universal  means  of  the  All-Wise 
for  protecting  organic  nature  from  harmful  effects  of  extreme  cold. 


150 


HEAT  AND  STEAM-ENGINE. 


FIG.  4. 


Expansion  of  Gases. 

.  Figure  4.—  Expansion  of  gases.  —  Gases  and  vapors, 
being  under  the  influence  of  repulsion  (23),  and  having  little  cohesion, 
expand,  for  equal  increments  of  heat,  much  more 
than  either  solids  or  ordinary  liquids. 

Invert  the  bulbed  tube,  F,  and  insert  its  mouth 
into  a  tumbler  of  water;  then,  by  heating  the 
bulb,  if  only  with  the  hand,  the  air  will  expand 
and  escape,  as  indicated  by  the  bubbles  in  the 
water.  If  the  heat  be  removed,  the  air,  cooling, 
will  contract,  and  water  will  rise  to  take  the  place 
before  occupied  by  the  air  that  has  escaped.  Hence, 
the  height  to  which  the  liquid  rises  in  the  graduated 
tube  will  indicate  the  amount  of  expansion  of  the 
air. 


.  The  general  laws  of  expansion  of 
gases  by  heat  : 

1.  All  gases  have  the  same  co-efficient  of  expansion 
as  common  air. 

2.  Tlie  co-efficient  of  expansion  remains  the  same, 
'whatever  may  be  the  pressure  to  which  the  gas  is  subjected. 

3.  Under  the  pressure  of  the  atmosphere,  the  co-efficient  of  expansion 
for  all  gases  may  be  considered  as  0.3666,  between  the  freezing  and  boil- 
ing points  of  water  ;  or  T-J-T  of  the  volume  at  32°  for  each  degree  of 
Fahrenheit's  scale. 

228.  Relation  between  compressibility  and  expansibil- 
ity. —  The  expansibility  and  compressibility  of  a  substance  increases 
with  the  temperature. 

Solids  expand  less  than  liquids,  and  are  less  compressible;  while 
liquids  are  less  expansible  and  compressible  than  gases. 

The  most  expansible  solids  are  generally  the  most  easily  compressed. 

229.  Density  of  gases.  —  Density  of  gases  and  vapors  is  com- 
pared, for  a  standard,  with  common  air,  when  the  barometer  stands  at 
30  inches,  and  thermometer  at  32°  F.,  air  being  called  1,  or  1.000. 

The  method  of  determining  the  density  of  a  gas  is,  in  principle,.  the 
same  as  for  the  density  of  liquids. 

The  density  of  air  being  1.0000,  that  of  hydrogen  is  0.0692  ;  nitrogen, 
0.9714;  oxygen,  1.1056;  carbonic  acid,  1.5290. 

Hydrogen  is  the  lightest  known  body. 


HEAT. 


151 


SPECIFIC     HEAT. 

230.  Figure  5.  —  Calorimetry,   or  the  measurement  of  the 
quantity  of  heat  which  different  bodies  absorb  or  emit  during  a  known 
change  of  temperature.    Let  AA  be  a  large  vessel  made  of  ice,  having  a 
heavy  slab  of  ice,  M,  for  a  cover.   Suppose  it  were  required  to  determine 
the  relative  capacity  of  water  and 

mercury  for  heat.  In  a  glass 
flask,  T,  put  one  ounce  of  water, 
and  raise  its  temperature,  say, 
to  200°  F.;  then  place  the  flask 
in  the  ice  vessel,  and  cover  it 
over.  Let  the  water  cool  down, 
as  it  will,  tQ  32°.  Now,  pour 
off  the  water  that  is  in  the 
ice-vessel  (which  has  come  from 
the  warm  water  melting  the 
ice),  and  measure  it.  Do  the 
same  with  an  ounce  of  mercury ; 
and  it  will  be  found  that  the 
mercury  melts  only  one  thirty- 
third  as  much  ice  as  the  water. 
And  as  the  relative  amount  of 
water  obtained  from  the  ice- 
vessel  shows  the  relative  amount 
of  heat  that  is  necessary  to  raise  an  ounce  of  water  and  an  ounce  of 
mercury  from  32°  to  200°  of  temperature,  it  follows  that  water  has  33 
times  the  capacity  of  mercury  to  absorb  heat  in  undergoing  a  given 
change  of  temperature. 

231.  Specific  heat  or  caloric  capacity.— The  amount  of  heat 
which  a  body  is  capable  of  absorbing,  as  above  described,  is  called  the 
specific  heat,  or  caloric  capacity  of  the  body. 

232.  The  unit  of  heat,  or  thermal  unit,  is  the  quantity  of 
heat  required  to  raise  a  pound  of  water  from  32°  to  33°  F. 


Standard  of  specific  heat. — From  such,  and  other  experi- 
ments, as  above  shown,  tables  have  been  formed  expressing  the  specific 
capacity  of  different  bodies  for  absorbing  heat ;  water  being  taken  as 
the  standard,  and  marked  1.000. 


152 


HEAT  AND  STEAM-ENGINE. 


TABLE    SHOWING     SPECIFIC    HEAT    OP    DIFFERENT    SUBSTANCES. 


Cobalt  ............  106.96 

Zinc  ..............  95.55 

Copper  ............  95.15 

Arsenic  ...........  81.40 

Silver  .............  57.01 

Gold  ..............  32.44 

Platinum  ..........  32.43 

Mercury  ..........  33.32 

Effect  of  specific  heat  of  water  on  climate.—  The 

universality  of  water  and  its  high  specific  heat,  or  its  great  capacity  to 
absorb  and  emit  heat,  greatly  modifies  the  rapidity  of  natural  transi- 
tions, or  sudden  changes  from  hot  to  cold  and  cold  to  hot  seasons. 


Water 1.000 

Ice 513 

Charcoal 414 

Sulphur 241 

Glass 203 

Diamond 147 

Iron 114 

Nickel . ,  109 


FIG.  6. 


Specific  heat  of  gases.  —  If  a  unit  of  weight  of  any  gas, 
allowed  to  expand  without  change  of  pressure,  is  heated  from  32° 
to  33°,  the  amount  of  heat  thus  absorbed  (measured  in  fractions  of  the 
unit),  is  called  the  specific  heat  under  a  constant  pressure  ;  but  if  the 
gas  be  not  allowed  to  expand,  then  the  amount  of  heat,  so  required,  is 
called  the  specific  heat  under  a  constant  volume. 

236.  Figure  6.—  Compression  of  air  and  other  gases 
diminishes  their  capacity  for  heat.  —  Let  the  figure  represent 
a  strong  cylinder  and  piston  ;  on  the  lower  side  of  the 
piston  let  there  be  placed  a  piece  of  tinder,  represented 
by  the  dots.  If  the  air  in  the  piston  be  suddenly 
compressed,  by  forcing  down  the  piston,  the  tinder  will 
be  ignited,  because  the  capacity  of  the  air  for  heat  is 
diminished  with  a  diminution  of  its  volume. 

The  variation  of  capacity  of  substances,  under  va- 
riations of  volume,  is  clearly  shown,  and  impressed  on 
the  mind  with  the  sponge  and  water  illustration. 
Thus  :  if  a  sponge,  which  has  imbibed  as  much  water 
as  it  can  hold,  be  compressed,  a  portion  of  water 
exudes,  just  as  the  air  in  the  cylinder  allows  a  portion 
of  the  heat  to  escape  when  pressure  is  made.  On 
relaxing  the  force  on  the  sponge,  and  allowing  it  to 
dilate,  it  will  take  up  an  increased  quantity  of  water; 
and  air,  when  suddenly  dilated,  has  its  capacity  for 
heat  increased,  and  vice  versa.  Or:  equal  volumes 
of  all  gases  (measured  at  the  same  temperature  and 
pressure),  set  free  or  absorb  the  same  quantity  of  heat 


HEAT. 


153 


when  they  are  compressed  or  expanded  the  same  fractional  part  of 
their  volume. 


Heat  applied  to  warming  apartments  partly  con- 
sumed in  expanding  the  air.—  When  air  is  heated,  where  it  is 
free  to  expand,  as  in  a  room,  only  about  five-sevenths  of  the  heat 
applied  is  expended  in  producing  elevation  of  temperature  ;  the  other 
two-sevenths  being  taken  up  by  the  expansion  of  the  air,  to  be  given 
out  again  as  the  air  contracts  by  cooling. 


Specific  heat  affected  by  change  of  state.— A  body 
in  the  liquid  state  has  a  greater  specific  heat  than  in  the  solid  form ; 
owing  to  the  fact  that  additional  heat  is  required  to  convert  the  solid 
into  a  liquid;  as,  in  the  above  table  (233),  it  will  be  seen  that  the 
specific  heat  of  water  is  nearly  double  that  of  ice. 

On  the  other  hand  again,  in  the  gaseous  condition  of  a  body,  its 
specific  heat  is  less  than  when  it  is  in  the  liquid  state. 

The  following  table  exhibits  the  dependence  of  the  specific  heat  on 
the  physical  state  of  the  substance : 

SPECIFIC    HEAT    OP    DIFFERENT    STATES    OF    BODIES. 


SUBSTANCES. 

SPECIFIC   HEAT. 

SOLID. 

LIQUID. 

GASEOUS. 

Water  

0.5040 

0.0833 
0.0562 
0.0541 
0.0314 

1.0000 

0.1000 
0.0637 
0.1082 
0.0402 
0.5475 
0.5290 

0.4805 
0.0555 

0*45*34 
0.4797 

Bromine  

Tin  

Iodine  

Lead  

Alcohol  

Ether  .  . 

See  latent  heat  and  change  of  state,  303. 


COMMUNICATION     OF     HEAT. 

239.  Heat  is  communicated  in  three  ways :  1st,  By  con- 
duction  (chiefly  in  solids) ;  2d,  By  convection,  or  circulation  (in  liquids 
and  gases) ;  3d,  By  radiation. 

Conductibility  of  Solids. 

240.  Conduction  of  heat — conductors  and  non-conduct- 
ors.— If  one  part  of  a  solid  be  heated,  as  one  end  of  an  iron  bar,  the 
heat  will  slowly  travel  along  from  particle  to  particle,  until  it  reaches  all 
parts  of  the  body.     This  is  called  conduction.     As  the  ultimate  atoms 


154 


HEAT  AND  STEAM-ENGINE. 


or  molecules  of  matter  (4  and  5)  are  not  supposed  to  be  in  actual  contact 
(22  and  35),  conduction  is  sometimes  called  interstitial  radiation. 

241.  Different  solids  conduct  heat  differently.  Hence 
some  are  called  good  conductors  ;  others,  bad  conductors.  Solids  con- 
duct heat  better  than  liquids,  and  liquids  better  than  gases. 


.  Fi&ure  7.—  Determination  of  the  conductibility  of 
solids.  —  There  are  many  ways  of  testing  the  relative  condnctibility  of 
solids.  Screw  into  a  copper  ball,  T,  three  metallic  rods,  A,  N,  H,  as 
copper,  brass,  and  iron,  of  equal  length  and  diameter  ;  form  their  outer 


FIG.  7. 


extremities  into  little  cups ;  in  each  of  these  place  a  bit  of  phosphorus. 
If  now  the  ball  be  heated  with  a  spirit-lamp,  the  heat,  conducted  along 
the  rods,  will  ignite  the  phosphorus,  as  shown  at  the  extremity  of  the 
rod,  N ;  the  best  conductor  firing  its  phosphorus  first,  and  the  poorest 
last. 

TABLE     OP    CONDUCTIBILITY     OP     SOLIDS     (TYNDALL). 

(Silver  being  rated  100.) 


Silver.. 100 

Copper 74 

Gold     53 

Brass 24 

Tin..  15 


Iron 

Lead 

Platinum 

German  Silver 

Bismuth 


Good  conductors  of  heat  are  also  good  conductors  of  electricity 


HEAT. 


155 


Musical  tones  caused  by  conduction. — Experiments 
have  shown  that  conduction  of  heat  produces  vibratory  motion  of  the 
conductor,  accompanied  by  musical  tones. 


Conductibility  varies  with  molecular  arrange- 
ment.— In  homogeneous  solids,  conductibility  is  equal  in  all  direc- 
tions. While  in  wood  and  crystals,  it  is  greater  in  some  directions 
than  others. 

In  wood,  it  is  found  that  there  are  three  unequal  axes  of  calorific 
conduction ;  the  principal  one  being  parallel  to  the  fibres  of  the  wood. 
The  heat-conducting  power  of  wood  bears  no  definite  relation  to  its 
density,  some  of  the  lightest  being  the  best  conductors.  Green  woods 
conduct  heat  better  than  dry. 

The  conductibility  of  a  body  is  diminished  by  being  pulverized,  or 
otherwise  minutely  divided;  as  marble,  powdered ;  or  wood,  worked 
into  saw-dust. 

245.  Figure  8.— Conduction  the  principle  of  the  safety- 
lamp  of  Davy. — The  blaze  of  the  lamp  is  surrounded  or  inclosed 
by  a  cylinder  of  metallic  wire  gauze,  shown  ,  pIG  g 

by  the  double-dotted  lines.  If  the  lamp  or 
lantern  be  carried  into  a  mine,  or  any  place 
where  there  are  inflammable  gases,  the  blaze 
of  the  burning  gas  within  the  gauze  cylinder 
will  not  communicate  itself  to  the  gas  with- 
out;  for  the  reason  that,  as  the  flame  passes 
(or  is  passing)  through  the  gauze,  the  wires 
conduct  away  a  sufficient  amount  of  its 
heat  to  reduce  the  temperature  of  the  blaze 
below  the  combustible  intensity,  which,  of 
course,  extinguishes  it. 

This  lamp  is  of  immense  value  in  mines, 
and  has  been  the  means  of  preventing 
many  explosions  and  much  destruction  of 
human  life. 

Conductibility  of  Liquids. 

246.  Conductibility  of  Liquids. — 

Formerly,  from  experiments,  it  was  con- 
cluded that  liquids  were  absolutely  non-con- 
ductive, but  later  experiments  prove  that 
they  do  conduct  heat,  but  only  to  a  very 
limited  degree — except  mercury  ;  which,  being  a  metal,  is  a  good  con- 
ductor. 


150 


HEAT  AND  STEAM-ENGINE. 


$47.  Figure  9.— Heat  in  liquids  not  equalized  by  con- 
duction.— When  water  is  to  be  heated  in  a  vessel,  it  is  indispensable 
that  the  heat  be  applied  at  the  lower  part  of  the  containing  dish  ;  as 
will  be  presently  explained. 

(For  explanation  of  this  figure,  see  253.) 

/     FIG.  9.  FIG. 


Figure  10.— Non-conductibility  of  liquids  shown 
by  experiments  with  water. — Place  in  the  bottom  of  a  wide  tube  a 
piece  of  ice  ;  fill  the  lower  portion,  B,  with  Hue,  the  intermediate  portion, 
W,  with  plain,  and  the  upper  portion,  Y,  with  yelloiv  water,  as  shown. 
Take  the  heavy  metallic  ring,  L,  heat  it  red-hot,  and  place  it  as  seen, 
just  above  the  level  of  the  plain  water ;  and  immediately  the  water  on 
the  outer  portion  of  the  column,  as  far  down  as  the  ring,  will  begin  to 
ascend,  and  in  the  central  portion  to  descend ;  as  shown  by  the  bent 
arrows ;  and  soon  it  will  boil  without  mingling  with  or  raising  the 
temperature  of  the  plain  water  below.  By  placing  the  ring  down  just 
above  the  blue  water,  the  plain  water  will  boil  and  mix  with  the  yel- 
low. If  the  ring  be  placed  still  lower,  a  portion  of  the  blue  water  can 
be  made  to  boil  and  mix  with  the  other  colors ;  while  yet  the  ice  re- 
mains unmelted ;  thus  proving  that  heat  in  water  does  not  descend 
below  the  point  of  applied  heat,  either  by  conduction  or  convection. 


HEAT.  157 

i 

Conductibility  of  Gases. 

.  Conductibility  of  gases.  —  Gases  are  even  more  non-con- 
ductive of  heat  than  liquids.  Heat  in  these  is  so  readily  diifused  by 
currents,  it  is  difficult  to  make  experiments.  Substances  which  inclose 
large  volumes  of  air  within  their  pores,  as  down,  feathers,  wool,  etc., 
are  very  poor  conductors  of  heat  ;  and,  in  the  economy  of  Nature,  are 
employed  as  non-conductors. 


.  Relative  Conductibility  of  moist  and  dry  air.— 

Air  filled  with  moisture  is  rendered  thereby  a  much  better  conductor 
than  dry  air,  in  the  ratio  of  230  to  80  ;  hence,  at  the  same  temperature, 
a  damp  atmosphere  seems  colder  to  the  senses  than  dry  air,  as  it  more 
rapidly  conducts  away  animal  heat. 

2ol.  Relative  Conductibility  of  solids,  liquids,  and 
gases  of  the  same  temperature.  —  We  would  be  burned  with  a 
rod  of  metal  heated  to  120°  F.,  but  not  scalded  by  water  at  150°,  while 
dry  air  has  been  endured  without  injury  even  as  high  as  300°. 

252.  The  philosophy  of  clothing,  as  relates  to  heat,  consists 
in  wearing  non-conducting  materials  when  the  heat  of  the  air  is  greater 
than  that  of  the  body  (98  F.).  to  shield  the  body  from  heat  without  ; 
and  when  the  air  is  colder  than  a  comfortable  temperature,  to  keep  the 
heat  from  escaping  from  the  body  ;  and  for  all  temperatures  between 
these  two  limits,  conductive  materials  should  be  worn,  to  allow  the 
heat  of  the  body  to  escape. 

CONVECTION    OF     HEAT. 

Convection  of  Liquids. 

253.  Convection  of  liquids.  —  Though  liquids  and  gases  do  not 
readily  conduct  heat,  yet,  owing  to  their  perfect  mobility  (88),  and  the 
fact  that  liquids  and  gases  are  made  lighter  by  heat,  they  are  readily 
heated  by  a  process  of  circulation,  called  convection  ;  illustrated  by 
Figure  9  (247). 

The  heat  being  applied  at  the  centre  of  the  bottom  of  the  contain- 
ing vessel,  an  upward  current  of  heated  particles  takes  place  through 
the  centre  of  the  water,  and  a  corresponding  downward  current  of 
colder  particles  supplies  the  place  of  the  rising  current.  The  down- 
ward current  will  take  place  where  the  body  of  the  water  is  coldest, 
which,  of  course,  in  this  case,  is  next  to  and  near  the  sides  of  the  ves- 
sel. The  system  of  bent  arrows  will  show  the  currents.  In  Fig.  10 
(248)  the  descending  currents  were  in  the  centre,  because  the  heat  was 
applied  to  the  sides  instead  of  the  bottom  or  centre  of  the  vessel. 


158 


HEAT  AND  STEAM-ENGINE. 


Thick  and  viscid  liquids 
stirred  when  heated. 


do  not  circulate  so  freely,  and  need  to  be 


'254-  Ocean  currents. — The  Gulf-Stream. — Different  parts 
of  the  ocean  being  subjected  to  unequal  heat,  causes  regular  and  con- 
stant currents  of  great  magnitude  and  extent,  which  are  modified,  in 
their  direction,  by  the  form  and  distribution  of  land  and  water,  and 
the  earth's  rotation  around  its  axis. 

The  water  becomes  heated  under  the  tropics  and  flows  off  to  the  north 
and  south,  conveying  heat  to  and  evaporating  it  in  the  colder  regions; 
while  colder  currents  flow  from  higher  latitudes  toward  the  equator. 
These  streams  exert  a  great  effect  in  equalizing  and  modifying  the 
temperature  of  the  regions  through  which  they  pass. 

The  Gulf-Stream  is  one  of  the  most  remarkable  of  these  currents. 
It  is  called  the  Gulf-Stream  because,  in  its  circuit,  it  sweeps  around 
into  and  out  of  the  Gulf  of  Mexico. 

255.  Figure  11.— Heating  buildings  by  convection  of 
fluids  in  pipes. — If  the  boiler,  W,  and  system  of  pipes  are  filled  with 

water,  and  heat  be  applied  to 
the  boiler,  by  a  suitable  fur- 
nace, the  heated  water  will  rise 
from  the  boiler  and  pass  up  into 
the  turns,  N,  of  the  pipe,  and, 
by  radiation,  becoming  cooled, 
will  flow  down  in  the  descend- 
ing pipe  into  the  boiler  again, 
as  indicated  by  the  system  of 
arrows.  The  furnace  is  placed 
in  the  cellar,  F  representing  the 
floor  of  the  room  above.  The 
pipes  are  filled  and  replenished 
through  the  opening,  L.  This 
is  different,  somewhat,  from 
heating  by  steam,  as  will  be 
hereafter  shown. 

Convection  of  Gases. 

256.  Convection  of  gas- 
es.—  Heat  is  distributed  in 
gases  by  circulation  in  the  same 
manner  as  in  liquids.  A  cur- 
rent of  heated  air  rises  above  a 


HEAT.  159 

lighted  candle,  and  currents  of  colder  air  rush  in  below  to  take  its 
place,  as  shown  by  the  arrows  about  the  flame  in  Fig.  37  (344). 

A  room  is  heated  principally  by  convection.  Furnaces  in  the  lower 
parts  of  houses  heat  the  upper  apartments  by  the  circulation  of  air, 
which  conveys  the  heat  to  and  imparts  it  at  the  desired  locality. 

257.  Heating  buildings  by  steam. — The  apparatus  for  this 
purpose   is   not   unlike   that   employed  for  heating  with   hot  water, 
Fig.  11  (255) ;  the  pipes  not  being  tilled  with  water  but  with  steam, 
and  water  in  the  boiler  carried  to  a  greater  temperature. 

OPERATION. — The  steam  is  condensed  in  the  pipes  above,  and  runs 
back  to  the  boiler  in  the  form  of  liquid.  Every  pound  of  water  con- 
verted to  steam  in  the  boiler  takes  up  (from  the  fire)  over  900  units 
of  heat  (232),  and  renders  it  latent  (303),  and  every  pound  of  steam 
condensed  in  the  pipes  gives  out  this  heat  into  the  rooms  above.  The 
water  is  thus  made  to  convey  the  heat  from  the  furnace  below  to  the 
apartments  above. 

2 58.  The  atmosphere  an  immense  steam  heating  ap- 
paratus.— The  intense  heat  of  the  tropical  regions  evaporates  an 
immense  quantity  of  water  and  passes  it  into  the  air,  freighted  with 
a  relative  amount  of  latent  heat.    This  vapor  is  carried,  by  atmospheric 
currents  (also  caused  by  heat,  262),  northward  and  southward  to  colder 
regions,  where,  by  the  cold,  it  is  condensed,  and  compelled  to  liberate 
its  latent  heat.    The  condensed  vapor  falls  as  rain — thus  watering  as 
well  as  warming  the  colder  parts  of  the  earth — and,  through  rivers, 
lakes,  and  oceans,  finds  its  way  back  to  the  equatorial  regions. 

Thus  the  atmosphere  acts  as  a  universal  steam  apparatus,  in  which 
the  atmospheric  currents  and  the  rivers  are  the  pipes ;  and  the  sun, 
the  furnace.  This  apparatus  excels  the  device  of  man ;  for  it  not  only 
warms  the  cold  regions,  but,  by  conveying  away  the  heat,  it  cools  the 
hot  regions,  and  universally  supplies  one  of  the  most  indispensable 
elements,  which  is  water. 

259.  Relation  of  air  to  the  earth  same  as  glass  to  a  hot- 
house.— A  hot-house  catches  and  entraps  the  heat  of  sunbeams.    The 
luminous  heat  (204)  from  the  sun  passes  readily  through  glass,  but,  after 
being  reduced  to  obscure  heat,  by  absorption,  radiation,  and  reflection, 
it  cannot  pass  back  through  the  glass ;  as  glass  (besides  being  a  poor 
conductor)  will  not  transmit  obscure  heat. 

The  watery  vapor  in  the  atmosphere,  while  it  quite  readily  allows 
the  passage  of  luminous  rays,  is  almost  opaque  to  obscure  heat.  Hence, 
the  atmosphere  may  be  considered  an  immense  hot-house  to  catch  and 
entrap  the  luminous  heat  of  the  sun. 

- 


160 


HEAT  AND  STEAM-ENGINE. 


WIND. 

260.  Definition. — Wind  is  air  in  motion.     The  winds  are  an 
illustration  of  convection  on  a  large  scale. 

261.  Kinds  of  wind. — 1.  Regular  winds  are  those  which  blow 
constantly  in  nearly  the  same  direction,  as  the  trade-winds  (262). 

2.  Variable  winds  are  those  which  blow  sometimes  in  one  and  some- 
times in  another  direction. 

3.  Periodical  winds   are  those  which  blow  regularly  in  the  same 
direction  at  the  same  seasons  of  the  year  or  hours  of  the  day,  as  the 
land  and  sea  breezes  (264). 

4.  Hurricanes  or  cyclones. 

5.  Tornadoes  or  whirlwinds. 

262.  Figure  12.— Cause  of  winds.— Trade-winds.— The  air 

in  the  region  of  the  equator,  being  rarefied  by  tropical  heat,  rapidly 

FIG.  12. 


ascends  and  passes,  in  the  upper  atmospheric  regions,  over  toward  the 
poles,  and  a  current  of  cold  air  sets  in,  in  the  lower  atmospheric  re- 
gions, from  the  north  and  south,  to  take  the  place  of  the  rising  cur- 
rent ;  as  illustrated  by  the  arrows. 


HEAT.  161 

These  currents,  toward  the  poles  above  and  toward  the  equator 
below,  would  be  due  north  and  south  were  it  not  for  the  rotation  of 
the  earth  from  west  to  east.  The  surface  of  the  earth  moves  faster 
at  the  equator  than  in  high  latitudes.  Hence  air,  coming  down  from 
the  poles,  is  continually  coming  to  places  where  the  earth  is  moving 
faster  and  faster,  and  it  therefore  lags  behind.  This  effect,  near  the 
equator,  is  so  great  that  the  currents  seem  to  blow  from  the  northeast 
and  southeast  ;  while  it  is,  really,  only  the  earth  sweeping  under  the 
currents  in  the  opposite  direction.  These  are  called  the  trade-winds. 

263.  Variable  winds.  —  The  direction  of  the  upper  currents  is 
just  the  opposite  of  the  lower  ones  (above  described).  In  intermediate 
latitudes,  as  our  own,  the  upper  currents,  becoming  cooled,  begin  to 
settle  down,  which  commingle  the  upper  and  lower  currents;  and, 
flowing  in  opposite  directions,  cause  the  extreme  variableness  of  the 
winds  of  our  climate. 

Variableness  of  winds  is  also  produced  by  more  local  causes,  as  hills, 
valleys,  mountains,  relation  of  land  and  water,  etc. 


264-  Lan(i  an(i  sea  breezes.  —  Owing  to  the  greater  specific 
heat  of  water  (234),  the  sea  becomes  less  heated,  during  the  day,  than 
the  land.  Hence,  toward  evening,  the  air  over  the  land  rises,  and  a 
surface  current,  called  the  sea-breeze,  sets  in  from  the  water.  At  night 
the  land  cools  faster  than  the  water,  and,  consequently,  toward  morning 
a  current,  called  the  land-breeze,  sets  in  from  the  land.  These  breezes 
are  most  marked  on  islands,  especially  in  tropical  regions.  The  change 
of  seasons  modifies  winds. 

265.  Hurricanes  or  cyclones.  —  These  are  distinguished  from 
other  tempests  by  their  extent,  power,  and  sudden  change  in  direction. 
They  revolve  around  an  axis,  upright  or  inclined,  while  they  move 
over  the  surface  of  the  earth.     Their  progressive  velocity  is  from  10  to 
40  miles  per  hour;  their  rotary  velocity  is  sometimes  100  miles  per 
hour.     In  diameter  they  vary  from  100  to  500  miles.     They  rotate  in 
a  direction  contrary  to  that  of  the  course  of  the  sun. 

266.  Tornadoes  or  whirlwinds  differ  from  hurricanes  chiefly 
in  extent  and  continuance;  being  rarely  more  than  a  few  hundred 
rods  in  breadth,  with  a  track  usually  not  more  than  twenty-five  miles 
in  length.     They  continue  but  a  few  seconds  ;  many  times  acting  with 
fearful  energy  ;  overturning  buildings,  uprooting  trees,  etc. 

Water-spouts  are  whirlwinds  filled  with  water  or  vapor,  instead  of 
leaves,  sticks,  dust,  etc. 

11 


162 


HEAT  AND  STEAM-ENGINE. 


267.  Physical  properties  of  winds. — Winds  are  hot,  cold, 
dry,  or  moist.    From  sandy  deserts,  they  are  hot;  from  the  sea,  in 
lower  latitudes,  they  are  warm  and  moist;   and  from  the  north  they 
are  cold  and  dry.     Our  northeast  winds  are  cold  and  moist,  because 
they  come  over  the  Atlantic  ocean.     The  Simoon  is  a  hot  wind  that 
blows  from  the  deserts  of  Africa.     Its  temperature  is  often  120°  F. 

268.  General  direction  or  frequency  of  different  winds. 

— In  the  Table  the  relative  frequency  of  different  winds  is  given,  the 
total  number  of  winds  in  each  country  being  1,000. 

FREQUENCY    OP    DIFFERENT    WINDS. 


COUNTRIES. 

N. 

N.  E. 

E. 

S.  E. 

s. 

s.  w. 

w. 

N.  W. 

England  

82 

Ill 

99 

81 

Ill 

225 

171 

120 

France 

126 

140 

84 

76 

117 

192 

155 

110 

Germany  ...... 

84 

98 

119 

87 

97 

185 

198 

132 

Denmark 

65 

98 

100 

129 

92 

198 

161 

156 

Sweden    .  . 

102 

104 

80 

110 

128 

210 

159 

106 

Russia  

99 

191 

81 

130 

98 

143 

166 

192 

North  America. 

96 

116 

49 

108 

123 

197 

101 

210 

FIG.  13. 


269.  Figure  13.— Anemome- 
ters.—  Pressure  of  winds.  - 

Anemometers,  of  which  there  are 
many  kinds,  are  instruments  for 
measuring  the  pressure  and  velocity 
of  winds. 

The  anemometer  here  shown  con- 
sists of  a  tube,  open  at  both  ends, 
bent  in  the  form  of  the  letter  TJ ;  the 
end  of  one  arm  being  bent  to  the 
horizontal  direction,  and  the  mouth 
widened  to  receive  the  wind;  the 
whole  being  nicely  adjusted,  as  a 
vane,  on  the  rod,  H  (which  is  screwed 
into  a  block  of  wood  or  other  sup- 
port), so  the  mouth  will  turn  to  the 
wind.  Fill  the  instrument  about  half 
full  of  water  and  it  is  ready  for  use. 

OPERATION. — The  wind  will  press 
upon  the  fluid,  depressing  it  in  one 
tube,  as  at  A,  and  elevating  it  in  the 
other,  as  at  L,  as  indicated  by  the 


HEAT. 


163 


several  arrows.  The  weight  of  the  water  standing  between  the  level 
of  A  and  L,  equals  the  pressure  or  force  of  the  wind,  but  not  Us 
velocity. 

270.  Velocity  of  winds. — The  velocity  of  winds  is  indicated  by 
the  force  or  pressure  which  they  exert. 

The  effect  of  moving  the  instrument  through  still  air  being  the  same 
as  if  it  were  at  rest  and  the  air  in  motion,  to  prepare  the  instrument 
for  testing  the  velocity  of  different  winds,  let  the  above  anemometer 
be  moved  through  still  air,  on  a  calm  day,  by  any  means,  as  on  a  car- 
riage or  an  open  rail-car,  at  various  rates  of  speed,  and  mark  the  rise 
of  the  fluid  for  the  different  velocities  on  the  tube,  as  shown  by  the 
graduated  scale. 

The  tube  is  diminished  at  the  bottom  to  check  the  undulations  of 
the  water. 

The  force  of  the  wind  is  as  the  square  of  the  velocity ;  and,  hence, 
the  velocity  is  as  the  square  root  of  the  force. 

The  velocity  of  winds  varies  from  that  which  scarcely  moves  a  leaf  to 
that  which  overthrows  a  forest. 

The  following  table  shows  the  corresponding  height  of  water,  velocity 
of  wind,  and  force  exerted,  upon  a  square  foot  of  surface. 


VELOCITY  AND  FORCE  OF  WIND. 


HEIGHT  OF 
WATER  IN 
INCHES. 

FORCE    OF    WIND 
IN  POUNDS. 

VELOCITY  PER 
HOUR  IN  MILES. 

COMMON  APPELLATIONS  OP 
SUCH  WINDS. 

1 

Hardly  perceptible. 

4 

Gentle  breeze. 

6 

Pleasant  wind. 

10 

Brisk  wind. 

i 

1.3 

15 

18 

j-  Very  brisk  wind. 

i 

2.6 

25.5 

High  wind. 

5.2 

36 

Very  high  wind. 

2 

10.4 

50 

Storm. 

3 

15.6 

62 

Great  storm. 

4 

20.8 

76 

5 

26. 

80 

Hurricane. 

6 

31.25 

88 

7 

36.5 

95.2 

8 

41.7 

101.6 

Violent  hurricane. 

9 

46.9 

108. 

10 

52.1 

113.6 

11 

57.3 

119.2 

12                62.5 

124. 

L(54 


UK  AT  AND  STEAM-ENGINE. 


FIG.  14. 


MEASUREMENT  OF  TEMPERATURES. 

Thermometers. 

Before  explaining  the  principles  of  radiation,  the  use.  construction, 
and  method  of  making  thermometers  will  be  explained. 

271.  Thermometers. — A  thermometer  (heat* measurer)  is  an  in- 
strument for  measuring  temperature  (200) ;   and  depends  upon  the 
principle  that  bodies  expand  when  heated,  and  contract  when  cooled. 
Thermometers  have  been   made  of  many  different   substances,  each 
being  selected  as  a  standard.     For  special  purposes  they  are  made 
either  of  solids,  liquids,  or  gases.     For  common  purposes,  however, 
mercury  is  preferred,  because  of  its  great  range  of  temperature  between 
its  freezing  and  boiling  points ;  and  because  it  affords  nearly  uniform 
increments  of  expansion  for  uniform  increments  of  heat ;  and  because 
it  does  not  vaporize  in  the  vacuum,  and  is  not  too  bulky. 

2 72.  Figure   14. —  Mercurial  thermometer. —  This  con- 
sists of  a  bulb  of  glass,  at  the  upper  extremity  of  which  is  a  narrow 

tube  of  uniform  bore,  hermetically 
sealed  at  its  upper  end,  as  shown 
by  F,  C,  and  R. 

The  bulb  and  part  of  the  tube 
are  filled  with  pure  mercury,  and 
the  whole  is  attached  to  a  frame, 
on  which  is  a  graduated  scale  for 
measuring  the  rise  and  fall  of  the 
mercury  in  the  tube. 

Of  the  ordinary  mercurial  ther- 
mometers there  are  three  kinds, 
Fahrenheit.  Centigrade,  and  Reau- 
mur ;  respectively  represented  in 
the  diagram  by  F,  0,  and  K.  The 
F.  is  mostly  employed  in  the 
United  States  and  England;  the 
C.,  in  France ;  and  the  R.,  in  Ger- 
many. The  principle  of  operation 
is  the  same  in  all ;  the  difference 
between  them  consisting  in  the 
graduation,  as  shown  in  the  figure. 

In  the  Fahrenheit,  the  inter- 
mediate space  between  the  freezing 
and  boiling  points  (of  water)  is  divided  into  180;  the  freezing  point 


HEAT. 


165 


being  marked  32°,  and  the  boiling  point,  212°.  The  zero  in  this  is  32° 
below  the  freezing  point. 

In  the  Centigrade,  the  freezing  point  (zero)  is  marked  0,  and  the 
boiling  point,  100°. 

In  the  Reaumur,  the  freezing  point  (zero)  is  also  marked  0,  but  the 
boiling  point  80°. 

All  degrees  below  zero  are  designated  by  prefixing  the  sign  minus  ( — ). 

273.  Conversion  of  thermometric  scales.  —  In  reading 
foreign  books,  it  is  often  necessary  to  convert  one  of  these  scales  into 
another ;  hence  the  following  rules  : 

The  Centigrade  is  reduced  to  the  Fahrenheit  by  multiplying  the 
given  number  of  degrees  by  9,  dividing  the  product  by  5,  and  adding 
32  to  the  quotient :  and, 

The  Reaumur  to  the  Fahrenheit  by  multiplying  by  9  and  dividing 
by  4,  and  adding  32  :  or, 

The  Fahrenheit  to  the  others  by  reversing  these  processes.  Exam- 
ples: 

Cent.  100  X    9  =  900  -=-  5  =  180  +  32  —  212°  Faht  FlG  15 

Eeau.   80  X    9  =  720  -:-  4  =  180  +  32  =  212°  Fah. 
Fah.   212  -  32  =  180  X  5  =  900  -5-    9  =  100°  Cent. 
Fah.   212- 32  =  ISO  X  4  =  720-r-    9=    80°  Reau. 


274.  Figure  15.— Method  of  making  a 
thermometer. — The  glass  bulb  and  tube  being 
provided  with  a  funnel  at  the  top,  as  shown  at  A,  is 
nearly  filled  with  mercury,  as  seen  at  N.  The 
whole  is  then  heated  till  the  mercury  boils,  thus  fill- 
ing the  tube ;  when  the  funnel  is  melted  off,  and  the 
tube  hermetically  sealed.  On  cooling,  the  mercury 
descends  to  some  point  of  the  tube,  as  shown  in  N, 
leaving  a  vacuum  at  the  upper  end.  It  only  remains 
to  graduate  it,  and  attach  a  suitable  scale. 


.  Standard  points  in  the  thermome- 
ter.— If  there  existed  a  natural  or  absolute  limit  to 
temperature,  either  of  heat  or  cold,  it  could  be  taken 
as  the  natural  zero,  from  which  the  thermometric 
scale  might  be  numbered,  either  upward  or  down- 
ward, from  it.  But,  as  there  is  no  such  natural  limit 
or  zero,  the  thermometric  scale  must  be  arbitrary. 
As  the  melting  point  of  ice  and  the  boiling  point  of  water,  under  cer- 
tain given  conditions,  are  always  the  same  (respectively  called  the 


166 


HEAT  AND  STEAM-ENGINE. 


freezing  and  boiling  points),  they  have  been  adopted  in  all  countries  as 
the  two  temperatures  with  reference  to  which  thermometric  scales  are 
constructed. 


276.  Figure  16. — Method  of  graduating  thermometers. 

Fixing  the  freezing  point. — To  fix   the  freezing  point  of  a 

thermometer,  thrust  the  bulb  and  part  of  the  stem  into  a  vessel  of 
powdered  ice,  as  exhibited  in  the  drawing,  which  will  contract  the 
mercury  down  to  the  temperature  of  the  melting  ice.  With  a  diamond 
mark  the  position  of  the  mercury  on  the  tube,  or  by  other  means,  on 
paper  previously  attached  to  the  tube. 

If  it  is  to  be  a  Fahrenheit  thermometer,  mark  this  point  32,  as 
shown;  if  a  Centigrade  or  Reaumur,  mark  it  0.  This  will  consti- 
tute the  freezing  point;  and  in  the  Centigrade  and  Keaumur,  the 
zero. 

FIG.  16.  FIG.  17. 


^77.  Figure  17.— To  fix  the  boiling  point  of  thermo- 
meters.—To  fix  the  boiling  point,  plunge  the  instrument  into  boiling 
water,  or,  what  is  more  accurate,  a  steam-bath,  by  means  of  a  vessel,  as 
seen  in  the  figure,  leaving  the  top  of  the  stem  or  tube  in  view,  as  shown 


HEAT.  167 

at  X.  After  the  mercury  ceases  to  rise  in  the  tube,  mark  its  position, 
as  in  the  last  case. 

If  it  is  to  be  a  Fahrenheit,  mark  it  212,  as  shown ;  if  a  Centigrade, 
100 ;  if  a  Keaumur,  80 ;  and  this  will  constitute  the  boiling  point. 

To  complete  the  graduation,  divide  the  space  between  the  freezing 
and  boiling  points  into  as  many  equal  parts  as  there  are  designed  to 
be  degrees  in  the  scale.  The  divisions  are  continued  both  above  and 
below  the  fixed  points.  The  figures  expressing  the  number  of  degrees, 
increase  upward  and  downward  from  zero  (0).  Prefix  the  sign  minus 
(  — )  to  all  degrees  below  zero  (276). 

278.  Tests  of  thermometers. — For  ordinary  uses  thermome- 
ters may  be  tested  by  thrusting  them  into  melting  ice  and  boiling 
water,  to  determine  if  the  freezing  and  boiling  points  are  correctly 
fixed  or  marked.     When  inverted,  the   mercury  should  fall  with  a 
sudden  click,  and  fill  the  tube,  thus  showing  the  perfect  exclusion  of 
air. 

279.  Sensibility  of  a  thermometer  is  of  two  kinds.    It  may 
indicate  very  small  differences,  or  it  may  be  very  sensitive  to  sudden 
changes  of  temperature.     The  former  obtains  when  the  bulb  is  large 
and  the  bore  of  the  stem  is  small ;  the  latter,  when  the  bulb  is  small 
and  the  glass  thin. 

280.  Limits    of  the    mercurial    thermometer. — Though 
mercury  is  by  far  the  most  available  thermometric  fluid,  yet  it  has  its 
limits ;  that  is,  it  boils   at   662°  above   zero,  and   freezes  at  39°,  F., 
below  zero.     Hence,  for  testing  temperatures  above  and  below  these 
points,  other  substances  must  be  employed/    For  degrees  above  the 
boiling  point  of  mercury,  pyrometers  are  used ;  for  degrees  below  the 
freezing  point  of  mercury,  alcohol  is  employed. 

Pyrometers  are  made  in  various  ways — the  principle  employed  in 
most  of  them  being  the  expansion  of  solid  metals  by  heat. 

281.  Spirit  thermometers. — Alcohol  has  never  been  frozen, 
and  is,  therefore,  generally  employed  for  the  estimation  of  low  tem- 
peratures.    For  this  purpose  it  is  usually  colored  red,  to  render  it  more 
visible. 

For  higher  temperatures,  however,  alcohol  cannot  be  employed, 
as  its  limit,  in  this  direction,  is  soon  reached ;  its  boiling  point  being 
174°  F. 

Air  thermometers  are  the  best,  because  most  accurate,  for  very  high 
temperatures. 


168 


HEAT  AND  STEAM-ENGINE. 


282.  Figure  18.—  Self-registering  thermometers.— It  is 

desirable,  sometimes,  to  ascertain  the  highest  or  lowest  temperature, 
FlG.  18>  or  both,  in  places  and  at  times  where  or 

when  it  is  impossible  or  inconvenient  to 
make  observations  of  the  instrument;  as 
in  deep  water,  or  in  the  night,  or  at  some 
inaccessible  distance. 

For  this  purpose,  self-registering  ther- 
mometers of  various  kinds  are  constructed. 
The  one  here  represented  consists  of  a 
single  tube  twice  bent,  having  a  long  cy- 
lindrical bulb,  filled  with  alcohol,  which 
reaches  down  the  small  tube  to  H,  where  it 
rests  on  mercury ;  the  mercury  extending 
down  the  small  tube  around  to  opposite  H ; 
above  this,  the  tube  is  filled  with  alcohol 
and  air ;  so  that  the  lower  part  of  the  tube, 
A,  is  filled  with  a  heavy,  and  the  upper  part 
with  a  light,  fluid.  Opposite  H  are  little 
iron  disks  or  floats,  made  to  work  in  the 
tube  with  a  little  friction,  by  means  of 
hair  springs,  which  keep  them  in  any 
given  place,  unless  moved  by  the  mercury ; 
and  past  which  the  alcohol  can  freely  pass. 
These  disks  are  put  in  contact  with  the 
mercury  (on  which  they  .float),  by  means  of  a  magnet  carried  along 
outside  of  the  glass. 

OPEBATIOI*. — When  the  alcohol  in  the  large  bulb  expands  by  heat, 
it  drives  the  mercury  and  floats  around  before  it ;  when  it  contracts  by 
cold,  the  float  on  the  right  hand  is  held  in  its  highest  position  by  the 
friction  spring,  while  the  mercury  is  driven  back  by  the  alcohol  and 
expansion  of  air  above  it.  When  the  alcohol  in  the  large  bulb  has 
contracted  by  cold,  and  then  again  expanded  by  heat,  the  float  on  the 
left  hand  will  be  left  at  its  highest  point. 

So,  after  the  instrument  has  been  submitted  to  different  tempera- 
tures, as,  for  instance,  at  some  place  in  a  distant  forest,  or  on  a  moun- 
tain, unobserved  for  a  whole  year,  it  can  then  be  inspected,  and  the 
position  of  these  floats  will  show  the  lowest  and  highest  temperature 
that  has  obtained  during  the  year. 

283.  Figure  19.— Differential  thermometers.  —  The  ob- 
ject of  these  instruments  is  to  determine  the  difference  of  temperature 


UK  AT. 


169 


of  two  different  points  or  substances.  The  one  here  represented  con- 
sists of  a  two-bulbed  tube,  bent  twice  at  right  angles;  the  tube  being 
partly  filled  with  alcohol  or  sulphuric  acid,  and  air  occupying  the  bal- 
ance of  the  space.  If  both  bulbs  are  equally  heated,  the  liquid  will 
stand  at  the  same  height  in  both  branches  of  the  tube ;  but  if  one,  as 


FIG.  19. 


A,  is  heated  more  than  the  other,  the  liquid  will  be  depressed  in  its 
branch,  as  at  N,  and  rise,  as  at  T,  in  the  other  branch,  as  indicated  by 
the  arrows,  until  the  tensions  in  the  two  bulbs  balance  each  other; 
and  the  graduated  scale  attached  to  one  of  the  arms  will  show  the 
difference  of  temperature  of  the  two  bulbs. 


RADIATION    OP     HEAT. 


Eradiation  of  heat.— 1st.  Hot  bodies  radiate  heat  equally 
in  all  directions. 

2d.  Radiated  heat  proceeds  in  straight  lines,  diverging  in  every  direc- 
tion from  the  points  where  it  emanates,  same  as  rays  of  light  from  a 
luminous  body.  These  lines  are  called  thermal  rays  or  heat  rays. 

It  is  the  radiant  heat  of  the  sun,  a  common  fire,  a  burning  lamp,  etc., 
that  warms  us. 


170  HEAT  AND  STEAM-ENGINE. 

285.  Cooling  by  radiation. — Thermal  rays  continue  to  issue, 
until  the  heat  of  the  body  sinks  to  the  actual  temperature  of  the  air, 
or  surrounding  medium  (203). 

Conduction  of  heat  may  be  internal  radiation  from  particle  to  par- 
ticle, termed  interstitial  radiation  (240). 

286.  Intensity  of  radiation.— The  intensity  of  radiant  heat  is 
according  to  the  following  laws : 

1st.  It  is  proportional  to  the  temperature  of  the  source. 

2d.  It  is  greater  in  proportion  as  the  rays  are  emitted  in  a  direction 
more  nearly  perpendicular  to  the  radiating  surface. 

3d.  It  is  inversely  as  the  square  of  the  distance  from  the  source. 

This  law  is  the  same  as  that  of  the  force  of  gravity  and  the  intensity 
of  light,  and  is  illustrated  and  explained  by  paragraph  47. 

287 .  Radiant  heat  is  partially  absorbed  by  the  medium 
through  which  it  passes,  but  is  not  sensibly  affected  by  the  motion 
of  the  media,  as  of  winds  in  air. 

The  sun's  rays  lose  about  one-third  of  their  heat  in  passing  through 
the  atmosphere,  the  remainder  being  absorbed  or  reflected  at  the  surface 
of  the  earth. 

288.  Radiation  in  vacuo. — Radiation  takes  place  more  freely 
in  a  vacuum  than  in  the  air. 

289.  Universal  radiation  and  constant  mutual  exchange 
of  heat  between  bodies. — Heat  is  radiated  from  all  bodies  at  all 
times,  whether  their  temperatures  be  the  same  as,  or  different  from, 
that  of  surrounding  bodies;  for  it  is  the  tendency  of  heat  to  place 
itself  in  equilibrium  (203).     Of  several  bodies  of  different  temperatures, 
the  hotter  ones  give  off  more  than  they  receive,  and  the  colder  ones 
absorb  more  than  they  give  off;  and  when  thus  equilibrium  is  restored, 
each  body  continues  to  give  off  and  absorb,  but  in  equal  quantities. 

ACTION  OF  DIFFERENT  BODIES  UPON  HEAT. 

Surface  Action. 

290.  Incident  heat  absorbed  and  reflected. — A  ray  of  heat 
falling  upon  the  surface  of  a  body,  is  divided  into  two  parts,  one  of 
which  enters  the  body  and  is  absorbed,  and  the  other  is  deflected  or 
bent  from  its  course.     This  bending  is  called  reflection.     The  laws  of 
reflection,  as  relate  to  the  angles  of  incidence  and  reflection,  are  the 
same  as  for  light,  sound,  and  motion  (57).     The  point  where  the  bend- 


HEAT. 


171 


ing  occurs  is  called  the  point  of  incidence ;  before  incidence  the  ray  is 
called  the  incident  ray  ;  after  incidence,  the  reflected  ray.  A  perpen- 
dicular to  the  reflecting  surface  at  the  point  of  incidence  is  termed  the 
normal.  The  angles  formed  by  the  incident  and  reflected  rays  with 
the  normal,  are  called,  respectively,  angles  of  incidence  and  reflection. 
The  plane  of  the  incident  ray  and  the  normal  at  the  point  of  incidence 
is  called  the  plane  of  incidence.  The  plane  of  the  normal  and  the 
reflected  ray  is  called  the  plane  of  reflection. 

291.  Figure  20. — Laws  which  govern  the  reflection  of 
heat. 

1st.  The  planes  of  incidence  and  reflection  coincide. 

2d.  The  angles  of  incidence  and  reflection  are  equal. 

Let  W  be  a  tin  box,  with  blackened  faces,  and  filled  with  hot  water ; 
T,  an  intercepting  screen,  provided  with  a  small  opening ;  A,  a  reflecting 
surface;  and  F,  a  differential  thermometer.  NL  is  a  normal  to  the 

FIG.  20. 


reflecting  surface.  The  surface,  J,  radiates  heat  in  all  directions,  but, 
by  means  of  the  screen,  T,  as  shown,  only  a  single  ray  is  permitted  to 
fall  on  the  reflector.  By  this  apparatus  it  is  demonstrated  that,  what- 
ever be  the  value  of  the  angle  of  incidence,  the  planes  JLN  and  NLF 
coincide  with  each  other,  and  the  angles  JLN  and  NLF  are  equal  to 
each  other. 


>.  Figure  21.— Reflection  of  heat  from  concave  mir 
rors. — This  figure  represents  two  concave  mirrors,  which  are  paraboli- 
cal in  shape  (392),  turned  face  to  face,  called  conjugate  mirrors.     The 
axis  of  such  mirrors  is  a  normal  to  the  surface  at  the  middle  point. 


172  HEAT  AND  STEAM-ENGINE. 

In  accordance  with  the  laws  just  explained,  rays  of  heat,  as  well  as  rays 
of  light,  parallel  to  the  axis  of  such  mirrors,  will  be  reflected  to  a  single 
point,  called  the  focus  of  the  .mirror ;  and,  conversely,  rays  radiating 
from  the  focus  will  be  reflected  in  lines  parallel  to  the  axis.  This  is 
demonstrated  by  placing  a  piece  of  inflammable  substance,  as  phospho- 
rus, in  the  cup,  A,  at  the  focus  of  the  mirror  on  the  right,  and  a  heated 

FIG.  21. 


cannon-ball  in  the  focus,  T,  of  the  other  mirror.  As  indicated  by  the 
lines,  arrows,  and  smoke,  the  heat  from  the  ball,  T,  will  be  reflected 
from  the  left-hand  mirror  to  the  right-hand  mirror  in  parallel  lines, 
and  again  reflected  to  the  cup,  igniting  the  phosphorus.  The  mirrors 
may  be  several  yards  from  each  other.  Single  parabolic  mirrors,  called 
burning  mirrors,  are  employed  to  collect  the  rays  of  the  sun. 

The  reflection  of  heat  in  vacuo  takes  place  according  to  the  same 
laws  as  in  air. 

293.  Reflective  power  of  different  substances. — Different 
bodies  possess  different  powers  of  reflection.  Some  substances  reflect 
more  and  absorb  less ;  others  absorb  more  and  reflect  less  than  others ; 
hence,  there  are  good  and  bad  absorbers.  Good  reflectors  are  bad 
absorbers  ;  and  bad  reflectors  are  good  absorbers. 

294-  Determination  of  reflective  power.— The  source  of 
heat  is  a  tin  canister,  F  (Fig.  22),  tilled  with  boiling  water.  The 
thermal  rays  are  converged  by  the  concave  mirror,  E,  and  thrown  upon 
a  small  plate  of  the  substance  to  be  tested,  by  placing  it  between  the 
mirror  and  focus,  so  that  the  rays  reflected  from  the  substance  shall 
fall  upon  the  bulb  of  the  differential  thermometer.  The  substance  is 
not  shown  in  the  figure. 

Polished  brass  possesses  the  highest  reflecting  power;  silver  reflects 
nine-tenths,  tin  eight-tenths,  glass  one-tenth  as  much  as  brass.  Plates 
blackened  by  smoke  do  not  reflect  heat  at  all. 


HEAT.  178 

295.  Figure  22.— Absorptive  power.— As  previously  stated, 
different  substances  possess  very  different  powers  of  absorbing  heat. 
The  absorptive  power  of  a  substance  is  in  the  reverse  ratio  of  its  reflec- 
tive power ;  the  best  reflectors  being  the  worst  absorbents ;  and  vice 

versa. 

FIG.  22. 


Let  the  source  of  heat  be  a  canister,  F,  of  boiling  water;  bring  the 
thermal  rays  to  a  focus  by  means  of  the  parabolic  mirror,  E,  as  repre- 
sented. In  the  focus  place  the  bulb  of  the  differential  thermometer, 
successively  covered  with  the  different  substances  to  be  tested. 

Substances  blackened  with  smoke,  or  covered  with  carbonate  of  lead, 
absorb  nearly  all  the  radiated  heat  thrown  upon  them  ;   glass, 
polished  cast-iron,  T2^  ;  tin,  TW;  silver,  T£¥. 


Absorptive  power  of  colors.  —  The  same  cloth  differently 
colored  has  different  absorptive  powers.  According  to  their  absorbent 
power,  the  colors  stand  in  the  following  order  ;  black  (warmest),  violet, 
indigo,  blue,  green,  red,  yellow,  and  white  (coldest).  Hence,  summer 
clothing  is  made  of  light-colored  and  winter  clothing  of  dark-colored 
fabrics. 

296.  Emission  or  radiating  power.—  The  emission  power 
of  a  body  is  its  capacity  to  emit  or  radiate  the  heat  it  contains.  To 
determine  the  emission  power  of  different  substances,  the  apparatus 
(Fig.  22)  above  described  is  employed,  —  The  different  sides  of  the 
canister  being  made  of  the  different  substances  to  be  tested,  as  tin, 
brass,  blackened  surfaces,  glass,  paper,  etc.  On  turning  these  different 
faces  toward  the  mirror,  the  thermometer  indicates  different  degrees 
of  temperature.  Experiments  show  that  radiating  powers  of  bodies 
are  the  same  as  their  absorbing  powers  ;  that  is,  a  good  radiator  is  a 
good  absorber,  but  a  bad  reflector  ;  and  vice  versa. 


174  HEAT  AND  STEAM-ENGINE. 

297.  Causes  which  modify  the  reflective,   absorbent, 
and  emission  powers  of  bodies. — These  causes  are  polish,  density, 
direction  of  the  incident  rays,  nature  of  the  source  of  heat,  and  color. 

1st.  Other  things  being  equal,  polished  bodies  are  better  reflectors 
and  worse  absorbers  than  unpolished  ones. 

2d.  Other  things  being  the  same,  dense  bodies  are  better  reflectors 
and  worse  absorbers  than  rare  ones. 

3d.  Other  things  being  equal,  the  nearer  the  incident  rays  approach 
the  direction  of  the  normal,  the  less  will  be  the  portion  reflected  and 
the  greater  the  portion  absorbed. 

4th.  The  nature  of  the  source  of  heat  sometimes  modifies  the  reflec- 
tive and  absorbent  powers.  For  example,  a  body  painted  with  white 
lead  absorbs  more  heat  from  a  canister  of  boiling  water,  than  though 
the  same  heat  were  emitted  by  a  lamp.  But  if  a  body  be  painted  with 
lamp-black,  the  amount  is  the  same,  whatever  be  its  source. 

5th.  Other  things  being  the  same,  light-colored  bodies  absorb  less 
and  reflect  more  heat  than  dark-colored  ones  (295). 

DIATHERMANCY  —  REFRACTION  —  POLARIZATION. 

298.  Transmission  of  radiant  heat. — Light  passes  through 
all  transparent  bodies,  from  whatever  source  it  may  come.     The  rays 
of  heat  from  the  sun  also  pass  through  transparent  substances.    Radiant 
heat,  however,  from  terrestrial  sources,  is  in  a  great  degree  arrested  by 
many  transparent  substances,  as  well  as  by  opaque  bodies.     For  exam- 
ple, window-glass  remains  cold  while  the  heat  of  the  sun  passes  through 
it,  but  the  same  glass  held  before  a  common  fire  arrests  a  large  part  of 
the  heat,  and  none  of  the  light.     Kock-salt,  however,  will  transmit  the 
heat  of  the  fire. 

Bodies  which  transmit  heat  are  termed  diathermanous,  or  diathermic 
(signifying,  through  and  to  heat). 

Many  substances  are  eminently  diathermic,  which  are  nearly  opaque 
to  light;  smoky  quartz,  for  example.  Hence,  solids  that  are  trans- 
parent to  light  do  not  necessarily  allow  the  passage  of  heat,  and  vice 
versa. 

Rock-salt  is  the  only  substance  that  transmits  an  equal  amount  of 
heat  from  all  sources.  This  substance,  therefore,  is  to  heat  what  glass 
is  to  light;  and,  hence,  Melloni  called  it  the  glass  of  heat. 

299.  Causes  which  modify  the  diather manic  power  of 
bodies,  are,  the  nature  of  the  source  of  heat,  the  degree  of  polish,  the 
thickness  and  number  of  the  screens  through  which  the  heat  has  been 
previously  transmitted. 


HEAT.  175 

300.  Diathermancy  of  the  air.— The  atmosphere  is  very  dia- 
thermanic.     If  it  were  not,  the  upper  layers  would  be  much  heated  by 
the  solar  rays  passing  through  them;  and  the  earth  would  receive 
correspondingly  less  heat  from  the  sun. 

301.  Figure  23.— Refraction  of  heat.— Heat,  like  light,  is 
refracted  or  bent  out  of  its  course  in  passing  obliquely  through  dia- 
thermanic  substances,  as  shown  by  the  figure,  illustrating  the  burning 

FIG.  23 


glass ;  which  consists  of  a  double  convex  lens.  By  such  a  lens  the 
rays  of  heat,  not  only  of  the  sun  but  other  heated  bodies,  are  concen- 
trated and  brought  to  a  focus,  same  as  rays  of  light.  Glass  lenses  are 
used  for  condensing  the  heat  of  the  sun,  but  they  will  not  condense  the 
heat  from  other  sources ;  besides,  they  would  themselves  become  heated. 
It  is  only  with  a  lens  of  rock-salt  that  heat  from  other  sources  than  the 
sun  can  be  condensed  by  refraction. 

Gunpowder,  paper,  and  other  combustibles  have  been  inflamed  with 
a  lens  of  ice. 

Lenses  of  thin,  pure  glass,  from  one  to  three  feet  in  diameter,  have 
volatilized  the  most  fixed  metals  at  the  focal  point,  and  fired  ships  and 
houses  at  a  considerable  distance. 

302.  Polarization  of  heat.— Heat  is  polarized  in  the  same 
manner  as  light  (513).  It  undergoes  double  refraction  by  Iceland 
spar,  and  the  two  beams  are  polarized  in  planes  at  right  angles  to  each 
other.  A  pencil  of  heat  polarized  by  a  plate  of  tourmaline  is  trans- 
mitted or  intercepted  by  another  tourmaline  plate,  under  the  same 
circumstances  that  a  pencil  of  polarized  light  would  be  transmitted  or 
intercepted. 

Polarization  of  heat  is  also  effected  by  reflection  from  plates  of  glass, 
and  by  repeated  refraction. 


!76  HEAT  AND  STEAM-ENGINE. 

CHANGE  OF  STATE  OF  BODIES  BY  THE  ACTION  OF  HEAT. 

Latent  Heat. — Liquefaction  and  Solidification. 

303.  Latent  heat  of  fusion. — During  the  conversion  of  a  solid 
into  a  liquid,  or  a  liquid  into  a  gas  or  vapor,  a  certain  quantity  of  heat 
is  absorbed  and  disappears,  so  that  the  thermometer  and  the  senses  give 
no  evidence  of  its  existence.  But  when  the  same  vapor  or  gas  is  again 
converted  to  a  liquid,  or  the  liquid  to  a  solid,  there  will  be  given  off 
and  rendered  evident  to  the  thermometer  and  senses,  or  set  free  from 
the  substance,  the  same  amount  of  free  heat  as  before  was  absorbed. 

This  heat,  thus  absorbed  or  rendered  insensible,  by  changing  matter 
from  a  dense  to  a  more  rarefied  form,  and  again  set  free  and  rendered 
sensible  by  changing  matter  from  a  rarefied  to  a  denser  form,  is  called 
latent  heat. 

EXAMPLE. — If  a  pound  of  pulverized  ice,  at  32°  F.,  be  mixed  with  a 
pound  of  water  at  174°,  the  heat  of  the  water  will  be  just  sufficient  to 
melt  the  ice ;  and  there  will  result  two  pounds  of  water  at  the  tem- 
perature of  32°.  Hence,  142°  of  heat  of  the  water  have  been  absorbed 
and  rendered  latent  in  converting  the  ice  to  water.  But  if  a  pound  of 
the  water  be  reconverted  to  ice,  then  the  142°  would  be  again  set  free. 
Hence,  we  say  the  latent  heat  of  water,  at  32°,  is  142°. 

304-  Liquefaction  and  solidification,  or  melting  and 
freezing;. — When  a  body  passes  from  a  solid  to  a  liquid  state,  it  is 
said  to  melt  or  fuse ;  and  the  act  of  conversion  is  called  fusion,  or 
liquefaction.  The  act  of  passing  from  the  liquid  to  the  solid  state  is 
termed  freezing ,  congelation,  or  solidification. 

Expansion,  the  first  effect  of  heat,  has  a  limit,  at  which  solids  become 
liquids.  The  force  of  cohesion  is  then  subordinate  to  the  power  of 
repulsion,  and  fusion  results. 

The  laws  of  liquefaction  and  solidification  are : 

1.  All  solids  enter  into  fusion  at  a  certain  temperature,  invariable  for 
the  same  substance. 

2.  Whatever  may  be  the  intensity  of  the  source  of  heat  when  the 
fusion  commences,  the  temperature  remains  the  same  until  the  whole 
mass  is  fused. 

3.  If  a  liquid  body  be  allowed  to  cool,  it  solidifies  at  the  same  tem- 
perature at  which  it  fuses. 

4.  The  temperature  of  a  body  remains  the  same  from  the  commence- 
ment to  the  end  of  its  solidification. 

5.  The  temperature  at  which  fusion  takes  place  is  different  for  dif- 
ferent bodies :  for  some  it  is  very  low ;  for  others,  very  high,  as  shown 
by  the  following  table. 


HEAT. 

MELTING    POINTS    OF    DIFFERENT    SUBSTANCES. 


^ 

177 


SUBSTANCE. 

TEMPERATURE. 

SUBSTANCE. 

TEMPERATURE. 

Mercury  

—39°  F. 

Bismuth 

507°  F 

Ice  

32° 

Lead 

635° 

Tallow  

91° 

Antimony 

842° 

White  Wax  

140° 

Zinc. 

933° 

Sulphur  

232° 

Silver 

1832° 

Tin  

442° 

Gold  

2192° 

Some  bodies  do  not  melt,  but  are  decomposed  by  heat,  as  paper, 
wood,  bone,  marble,  etc.  Bodies  composed  of  a  simple  element,  or  but 
one  kind  of  matter,  always  melt ;  though  carbon  has  resisted  all 
attempts,  as  yet,  to  fuse  it. 

Substances  difficult  of  fusion  are  called  refractory  bodies. 

305.  Peculiarities  in  the  fusion  of  certain  solids.— Cer- 
tain solids  soften  before  they  melt ;  as  tallow,  butter,  wax,  etc.     This 
is  because  they  are  composed  of  several  substances,  which  melt  at 
different  temperatures. 

Metals  that  are  capable  of  being  welded,  as  iron  and  platinum,  soften 
before  they  fuse. 
Glass  and  certain  metals  never  attain  perfect  fluidity. 

3 06.  Melting  and  freezing  always  gradual,  owing  to  the 
absorption  or  evolution  of  heat  during  these  processes. 

As  solids  cannot  pass  into  the  liquid  state  without  absorbing  and 
rendering  latent  a  great  amount  of  heat  (303),  the  very  act  of  melting 
deprives  the  immediately  surrounding  air  of  so  much  of  its  heat  as  to 
partially  arrest  the  melting  process.  And  as  the  act  of  freezing  liber- 
ates latent  heat,  the  freezing  body  becomes  surrounded  with  a  layer  of 
warm  air,  and  thus  the  process  of  freezing  is  partially  arrested  by  the 
very  freezing  itself.  Hence  the  seeming  paradoxes,  that  melting  is  a 
cooling  process,  and  freezing,  a  warming  process.  Yet  it  is  true,  that 
all  processes  of  freezing  are  processes  of  warming,  and  all  processes  of 
melting  are  processes  of  cooling. 

Were  this  not  the  case,  melting  and  freezing  would  be  instantaneous, 
and  so,  dangerous.  Water,  at  32°,  would  immediately  become  ice; 
and  ice  and  snow  would,  by  a  slight  increase  of  temperature,  instan- 
taneously return  to  water,  causing  destructive  freshets,  etc. 

3 07.  Why  ice  does  not  acquire  great  thickness.— It  is 

owing  to  this  law  of  absorption  and  liberation  of  heat,  by  melting  and 

12 


178  HEAT  AND  STEAM-ENGINE. 

freezing  of  water,  together  with  the  law  of  its  irregular  expansion 
(223)  and  its  high  specific  heat  (234),  that  ice  never  acquires  any  very 
great  thickness. 

308.  Latent  heat  of  water  graduates  the  changes  of 
temperature. — It  is  also  owing  to  the  law  of  absorption  and  libera- 
tion of  heat,  by  melting  and  freezing  of  water,  together  with  the  law 
of  its  irregular  expansion  (223)   and  its  high  specific  heat  (234),  that 
we  have  such  a  gradual  and  healthful  approach  of  hot  and  cold  seasons. 
In  autumn  the  water  has  142°  of  heat  to  give  out  before  it  solidifies ; 
in  the  spring  it  must  receive  the  same  amount  before  it  will  melt; 
serving  as  a  check  upon  the  sudden  changes  of  temperature. 

309.  Freezing    mixtures. — These  are  made  in   accordance 
with  the  above  law  of  absorption  of  heat.     Salt  and  pounded  ice,  for 
instance,  mixed  together,  and  acting  upon  each  other   to  mutually 
hasten  their  liquefaction,  will  so  rapidly  absorb  heat  from  surrounding, 
bodies  (cream  for  example)  as  to  freeze  them. 

In  a  mixture  of  salt  and  snow,  the  thermometer  may  be  reduced  to 
0,  F. 

310.  Crystallization. — When  bodies  pass  slowly  from  the  liquid 
to  the  solid  state,  their  particles,  instead  of  arranging  themselves  in  a 
confused  manner,  tend  to  group  themselves  into  regular  forms,  called 
crystals,  by  a  process  termed  crystallization  (25). 

Sugar-candy,  alum,  common  salt,  and  snow-flakes,  are  examples  of 
crystallized  bodies. 

VAPORIZATION. 

311.  Definitions. — Vaporization.— A  liquid  sufficiently  heated 
is  converted  into  the  gaseous  form,  and  is  called  a  vapor.    This  change 
of  state  is  called  vaporization. 

Conversely,  if  heat  be  abstracted  from  a  vapor  it  will  return  to  the 
liquid  form.  This  change  of  a  vapor  to  a  liquid  is  called  condensation. 

Thus  water,  at  212°  R,  is  rapidly  converted  into  steam,  an  invisible 
vapor. 

Vapors  are  generally  colorless,  and  endowed  with  an  expansive 
force  or  tension  ;  which,  when  heated,  may  become  very  great. 

Boiling  or  ebullition  is  the  rapid  formation  of  vapor  throughout  the 
whole  mass,  producing  agitation. 

Evaporation  occurs  gently  and  invisibly,  only  at  the  surface  of 
liquids,  as  on  the  surface  of  water  in  an  open  dish. 

Sublimation  is  the  change  of  solids  to  vapors  without  the  interme- 


HEAT. 


179 


diate  liquid  condition;  such  as  camphor,  iodine,  musk,  and  odorous 
bodies  generally. 

3 12.  Volatile  liquids  and  fixed  liquids.—  Volatile  liquids 
are  those  which  have  a  natural  tendency  to  pass  into  a  state  of  vapor  at 
ordinary  temperatures ;  such  as  alcohol,  ether,  essences,  essential  oils, 
turpentine,  and  the  like. 

Fixed  liquids  are  those  which  do  not  pass  into  a  vapor  at  any  tem- 
perature ;  as,  for  example,  fish-oils,  olive-oils,  and  the  like ;  which,  at 
high  heat,  are  decomposed  into  various  gases,  but  to  no  true  vapors 
that  can  be  again  condensed  into  the  original  liquid. 

313.  Latent  heat  of  evaporation. — A  large  amount  of  heat 
disappears,  or  is  rendered  latent,  during  evaporation ;    and  is  again 
liberated,  or  set  free,  by  condensation.     See  latent  heat  of  fusion  (303). 

3 IJf-  Latent  heat  of  steam. — The  amount  of  heat  absorbed  or 
rendered  latent  in  converting  ice  to  water  is  142°  F.  (303) ;  and  in 
converting  water  to  steam  there  is  absorbed  or  rendered  latent 


315.  Latent  and  sensible  heat  of  steam  at  different 
temperatures.  —  The  whole  amount  of  heat  in  steam  is  the  latent 
heat  plus  the  sensible  heat.  Thus  the  heat  pIG  24. 

of  steam  at  the  temperature  of  ebullition 
is  967°.o  +  212°  =  1179°.5  F.  The  heat 
absorbed  in  evaporation  is  less  as  the  tem- 
perature of  the  vaporizing  liquid  is  higher. 

Experiments  show  that  the  sum  of  the 
late  tit  and  sensible  heat  of  steam  increases 
with  the  temperature,  by  a  constant  differ- 
ence of  y^g-  of  a  degree  for  each  degree  F. 


Ebullition  or  Boiling. 

31  6.  Figure    24.  —  Ebullition.  — 

Ebullition  or  boiling  is  a  rapid  evaporation 
in  which  the  vapor  escapes  in  the  form  of 
bubbles.  The  bubbles  are  formed  in  the 
interior  of  the  liquid  (represented  by  the 
specks  between  the  bottom  of  the  liquid 
and  its  surface),  and,  rising  to  the  surface, 
they  collapse,  permitting  the  vapor  to  pass 
into  the  air.  The  cloudy  vapor  seen  above 


180 


HEAT  AND  STEAM-ENGINE. 


the  vessel,  commonly  called  steam,  is  not  steam,  but  minute  globules 
of  water ;  steam  itself  being  invisible. 

317 .  Laws  that  govern  the  phenomena  of  ebullition. 

1.  Under  the  same  pressure,  each  liquid  boils  at  a  fixed  temperature. 
The  temperature  at  which  a  liquid  boils  is  called  its  boiling  point. 

The  boiling  point  is  very  different  for  different  liquids ;  that  of  pure 
water  is  212°  F.,  when  the  pressure  on  its  surface  is  equal  to  30  inches 
of  mercury;  in  other  words,  when  the  barometer  stands  at  30  inches. 
The  boiling  point  of  ether  is  96° ;  alcohol,  174° ;  and  mercury,  662°  F. 

2.  The  pressure  remaining  the  same,  a  liquid  cannot  be  heated  higher 
than  the  boiling  point. 

The  additional  heat  does  not  raise  the  temperature,  either  of  water 
or  steam,  above  212°,  for  the  reason  that  it  becomes  latent  in  the 
steam  (314). 

Causes  Modifying  the  Boiling  Point. 

318.  Figure  25.— Variation  of  pressure  on  the  surface 
of  the   liquid  varies    the  boiling  point.  —  This  is  because 

ebullition  consists  of  the  formation  of  a  vapor 
of  the  same  elasticity  of  the  superincumbent 
atmosphere  or  pressure.  To  prove  that  this  is 
the  case,  fill  a  glass  flask  half  full  of  water,  and, 
having  caused  the  water  to  boil,  remove  the 
flask  from  the  fire  or  lamp,  and,  in  a  moment 
or  two,  cork  it  tight,  and  then  plunge  it  into  a 
vessel  of  cold  water,  as  represented  in  the  fig- 
ure, when  the  water  will  again  begin  to  boil, 
and  will  continue  to  do  so  until  the  tempera- 
ture is  reduced  quite  low. 

The  reason  why  water  boils  by  the  applica- 
tion of  cold  is,  that  the  steam  which  filled  the 
space  in  the  flask  above  the  water  is  (by  the 
cold   water)   condensed;   thereby   producing  a 
partial  vacuum,  which  diminishes  the  pressure 
on  the  surface  of  the  water;  thus  proving  that 
liquids  under  less  pressure  boil  with  less  heat. 
This  is  shown,  also,  by  placing  water,  heated  to  less  than  212°,  under 
the  receiver  of  an  air-pump ;  where  it  will  begin  to  boil  when  the  pres- 
sure of  the  air  is  partially  removed  by  the  pump. 

If  the  pressure  be  increased  the  temperature  of  the  boiling  point  will 
be  raised  (334). 


FIG.  25. 


HEAT.  181 

319.  Useful  applications  of  boiling  water  under  dimin- 
ished pressure  are  made  in  concentrating  vegetable  extracts,  cane- 
juice  (sugar),  etc.,  and  consequently  at  a  femperature  below  that  which 
would  injure  the  substances  treated. 

Boiling  point  affected  by  altitude. — On  ascending  moun- 
tains, the  boiling  point  of  liquids  falls,  because  the  atmospheric 
pressure  is  less ;  and,  conversely,  on  descending  into  mines,  etc.,  it 
rises  (134).  Experiments  prove  that  a  difference  of  about  543  feet  in 
elevation  produces  a  variation  of  1°  F.  in  the  boiling  point  of  water. 

It  is  impossible  to  cook  meat,  by  boiling)  on  high  mountains. 

Figure  26. — Franklin's  pulse  glass  is  a  further  illustration 
of  the  law.  that  liquids  under  less  pressure  boil  with  less  heat.  This 
consists  of  a  glass  tube  terminating  with  bulbs,  as  shown,  and  partly 

FIG.  26. 


filled  with  ether,  or  water,  and  sealed  while  the  liquid  is  boiling.  When 
the  liquid  is  cooled,  the  space  above  the  fluid  will  be  a  vacuum ;  then, 
if  only  the  heat  of  the  hand  be  applied  to  one  bulb,  the  liquid  will  be 
set  to  boiling  in  the  other,  as  indicated  in  the  drawing. 

320.  Solids  in  solution  in  liquids  raise  their  boiling 
points  in  proportion  to  the  quantity  dissolved.     For  example,  water 
which  holds  in  solution  as  much  common  salt  as  it  is  capable  of  dis- 
solving, requires  227°  F.  to  raise  it  to  the  boiling  point. 

If,  however,  the  body  dissolved  is  more  volatile  than  water,  then  the 
boiling  point  is  lowered. 

321.  The  nature  of  the  vessel  varies  the  boiling  point.— 

When  the  interior  of  the  vessel  is  rough,  the  projecting  points  form 
centres  for  developing  vapor,  and  the  boiling  point  is  lower  than  when 
the  surface  is  smooth.  Water  boils  at  a  lower  temperature  in  iron  than 
in  glass  vessels. 


182 


HEAT  AND  STEAM-ENGINE. 


Evaporation. 

322.  Evaporation  takes  place  slowly  in  the  open  air,  owing 
chiefly  to  atmospheric  pressure.  If  the  pressure  be  partially  removed, 
evaporation  will  take  place  more  rapidly;  if  wholly  removed,  it  will 
occur  instantaneously,  like  the  flash  of  gunpowder ;  especially  if  the 
liquid  is  very  volatile. 


Figure  27.— Evaporation  in  a  vacuum  takes  place  in 
obedience  to  the  following  laws : 

1st.  All  volatile  liquids  volatilize  instantly. 

2d.  At  the  same  temperature  the  vapors  of  different  liquids  possess 
unequal  elastic  force  or  tension. 

The  figure  represents  four  barometer  tubes,  filled  with  mercury,  and 
inverted  in  a  cistern  of  mercury,  with  a  graduated  scale  in  the  centre. 
37  The  mercury  will  stand  at  the  same  height 

in  all  the  tubes,  say  at  the  height  of  that  in 
the  one  on  the  left,  as  shown  by  the  arrow. 

If  now  a  drop  of  ether  passes  up  the  right- 
hand  tube  to  the  top  of  the  mercury,  it  in- 
stantly flashes  into  vapor  and  depresses  the 
mercury  half  its  height  or  more,  as  shown 
by  the  arrow;  which  illustrates  the  first  law. 
A  drop  of  alcohol  introduced  into  the  next 
tube  will  also  be  suddenly  converted  to  va- 
por, and  depress  the  mercury;  and  so  with 
a  drop  of  water  in  the  next  tube.  The  dif- 
ferent heights  of  the  columns  will  show  that 
the  three  several  fluids  possess  different  de- 
grees of  volatility  and  elasticity ;  thus  prov- 
ing the  second  law. 

If  more  ether  be  introduced  until  it  re- 
mains on  the  mercury  and  ceases  to  further 
evaporate,  it  is  said  to  have  reached  its  point 
of  saturation,  or  maximum  tension,  or  limit 
of  tension.  In  this  case,  the  tension  of  the 
vapor  balances  the  tendency  of  the  liquid  to 
pass  into  a  state  of  vapor.  Yet  the  quantity 
of  watery  vapor  necessary  to  saturate  a  given  space  is  always  the 
same,  whether  that  space  is  a  vacuum,  or  whether  it  contains  air  or 
any  other  gas. 

Heat  will  increase  and  cold  diminish  the  tension,  as  illustrated  by 
the  next  figure :  hence, 


HEAT. 


183 


FIG.  28. 


TJie  limit  of  tension  of  a  given  vapor  varies  with  the  temperature. 

The  amount  of  vapor  required  to  saturate  a  given  space  also  varies 
with  the  temperature. 

Rain  is  caused  by  the  vapors  (which  are  less  dense  than  the  air  in 
low  regions)  rising  to  regions  where  they  are  condensed  by  the  colder 
air. 

The  tension  of  different  vapors  varies.  Ether,  for  example,  being  25 
times  as  great  as  water,  and  6  times  that  of  alcohol. 

3%4*  Figure  28.— Evaporation  under  pressure. — Let  the 

end  of  the  short  arm  of  the  bent  tube  be  closed,  and  leave  the  other 
end  open,  and  fill  the  tube  two-thirds  full  of 
mercury,  which,  of  course,  will  completely  fill 
the  short  arm.  If,  now,  a  drop  of  ether  be  in- 
troduced into  the  top  of  the  short  arm,  A,  the 
pressure  of  the  air  on  the  mercury  in  the  long 
arm  will  prevent  its  evaporation  at  any  ordinary 
temperature.  If,  however,  the  tube  is  plunged 
into  a  vessel  of  water  heated  to  112°  F.,  the  ether 
will  be  converted  into  vapor  and  occupy  a  cer- 
tain portion,  as  AT,  of  the  tube,  holding  in 
equilibrium  the  pressure  of  the  atmosphere,  to- 
gether with  the  weight  of  the  mercurial  column 
whose  height  is  TN. 

If  the  tube  be  withdrawn  and  cooled,  the  va- 
por will  return  to  the  liquid  form  again. 

325.  Heat   increases    and    cold   de- 
creases the  tension  of  vapors.— The  illus- 
tration and  proof  of  this  principle  have  just  been 
given  (324). 

The  evaporation  of  liquids,  however,  takes 
place  at  temperatures  much  below  their  boiling 
points.  Even  at  ordinary  temperatures,  water, 

many  liquids,  and  some  solids,  vaporize.     Mercury,  whose  boiling  point 
is  as  high  as  662°  F.,  evaporates  at  all  temperatures  above  60°. 

326.  Causes  that  accelerate  evaporation.— The  principal 
causes  which  influence  the  amount  and  rapidity  of  evaporation  are: 

1.  Pressure. Increased  pressure  diminishes  evaporation.  The  ra- 
pidity of  evaporation  is  inversely  as  the  pressure  upon  the  surface  of 
the  liquid.  Were  the  pressure  of  the  air  entirely  removed,  many 
liquids  would  assume  a  permanently  aerial  form. 


184  HEAT  AND  STEAM-ENGINE. 

2.  Increased  surface  facilitates  evaporation. 

3.  Increased  temperature  increases  evaporation. 

4.  Diminished  quantity  of  vapor  in  the  air,  or  dry  air,  facilitates 
evaporation. 

5.  Renewal  or  circulation  of  the  air  over  the  fluid  accelerates  evapora- 
tion by  constantly  carrying  away  the  saturated  air,  and  allowing  drier 
air  to  take  its  place. 


.  Causes  of  condensation.  —  Condensation  of  vapor  is  its 
change  from  a  vaporous  to  a  liquid  state.  There  are  three  causes  of 
condensation  :  chemical  action,  pressure,  and  diminution  of  tempera- 
ture. 

1.  Chemical  action.  —  The  affinity  (2)  of  some   substances  for  the 
vapor  of  water  is  so  strong  that  they  absorb  it  from  the  air,  even  when 
the   latter  is   not   saturated  ;   such  are  quick-lime,  potash,  sulphuric 
acid,  and  others. 

2.  Pressure.  —  That  pressure  will  condense  vapor  is  shown  by  com- 
pressing a  volume  of  saturated  air. 

3.  That  a  diminution  of  temperature  causes  condensation  is  shown  by 
the  escape  of  steam,  or  breath,  into  cool  air,  which  condenses  the  vapor 

into  minute  globules  of  water.  This  principle 
is  shown,  too,  by  Fig.  28  (324)  ;  also  by  the 
accumulation  of  water  in  warm  weather,  on 
the  outside  of  a  pitcher  filled  with  ice-water. 

328.  Dew-point.  —  If    air,     saturated 
with   moisture,  is.  cooled,  a  portion  of  the 
moisture  will  be  precipitated  as  dew.     The 
dew-point  is  the  temperature  at  which  this 
deposition    takes    place.      The    more   fully 
the  air  is  saturated  with  moisture,  the  nearer 
the  dew-point  is  to  the  temperature  of  the 
atmosphere. 

329.  Figure  29.—  Pressure  exerted 
by  steam  or  heated  vapor.  —  This  figure 
illustrates  the  pressure  of  heated  vapor  by 
confining  the  vapor  over  the  surface  of  the 
evaporating  liquid. 

Fill  the  flask  about  two-thirds  full  of 
water,  and  pass  a  tube,  L,  through  the  stop- 
per, so  it  will  extend  nearly  to  the  bottom 
of  the  vessel.  Let  the  steam  or  vapor  be 


HEAT.  185 

generated  faster  than  it  can  escape  at  the  nozzle  of  the  pipe ;  then,  as 
the  generation  of  steam  goes  on,  the  increased  pressure  will  be  shown 
by  the  increased  force  with  which  the  water  will  rush  out  of  the  nozzle, 
by  the  elastic  force  of  the  steam  exerted  on  the  surface  of  the  liquid ; 
indicated  by  the  arrows.  It  is  this  elastic  force  of  steain  that  consti- 
tutes the  power  of  high-pressure  steam-engines. 

330.  Figure  30.— Candle-bombs,  illustrating  the  explo- 
sion of  steam-boilers. — These  are    globules  of  glass,  about  the 
size  of  a  pea,  with  a  neck  an  inch,  or  so,  long,  pIG 

in  which  a  drop  of  water  is  confined  by  her- 
metically sealing  the  neck.  When  one  of  these 
is  stuck  into  the  wick  of  a  lamp,  as  shown,  the 
heat  vaporizing  the  water,  and  there  being  no 
passage  for  the  escape  of  the  steam,  the  bulb  is 
burst  to  pieces  with  a  loud  explosion.  The 
mechanical  force  is  wonderful,  when  it  is  con- 
sidered how  little  water  is  employed.  This  is 
a  miniature  of  what  takes  place  in  the  burst- 
ing of  high-pressure  steam-boilers. 

33 1.  Spheroidal  state  of  liquids.— 

If  a  few  drops  of  water  be  poured  upon  a  red-hot  metal  plate,  as  a  com- 
mon fire-shovel,  they  gather  into  a  globule  which  rolls  about  without 
boiling,  or  coming  into  contact  with  the  plate.  In  this  condition  the 
water  is  said  to  be  in  the  spheroidal  state. 

The  temperature  of  the  plate  is  higher  and  that  of  the  spheroid 
lower  than  the  boiling  point  of  the  Uquid.  The  temperature  of  the 
vapor  from  the  spheroid  is  nearly  the  same  as  that  of  the  plate.  When 
the  temperature  of  the  plate  falls  to  a  certain  point,  the  liquid  will  come 
into  contact  with  it,  and  burst  into  ebullition,  and  quickly  evaporate. 

Causes  of  the  spheroidal  state  of  liquids. — 1.  The  repul- 
sive force  of  heat  exerted  between  the  plate  and  liquid.  2.  The  hot 
plate  converts  a  portion  of  the  liquid  into  vapor,  upon  which  the  sphe- 
roid rests.  3.  The  vapor,  being  a  poor  conductor,  prevents  the  conduc- 
tion of  heat  from  the  plate  to  the  spheroid.  4.  Evaporation  carries  off 
the  heat  as  it  is  absorbed  by  the  liquid,  which  assists  in  preventing  it 
from  entering  into  ebullition. 

It  is  in  accordance  with  these  principle,  that  the  hand  may  be 
bathed  without  harm  in  molten  metals.  The  moisture  of  the  hand, 
assuming  the  spheroidal  state,  prevents  immediate  contact  between  the 
hand  and  metal. 


186 


HEAT  AND  STEAM-ENGINE. 


FIG.  31. 


332.  Figure  31.— Condensation  of  steam.— As  a  vapor  will 
bear  no  increase  of  pressure,  so  neither  will  it  suffer  any  reduction  of 

temperature,  without   instantaneously  con- 
densing. 

Pour  some  water  in  the  vessel,  S,  pro- 
vided with  an  open  tube,  F,  and  stopper,  as 
shown ;  set  the  water  to  boiling  by  means 
of  a  spirit-lamp ;  and  when  the  air  has  been 
expelled  by  the  steam,  remove  the  flask 
from  the  lamp,  and  insert  the  lower  end 
of  the  tube  into  cold  water,  in  the  vessel,  W. 
The  vapor  or  steam  in  the  tube  will  be  con- 
densed by  the  cold  water,  causing  a  vacuum, 
and  the  atmospheric  pressure  will  force  the 
water  from  W  into  the  pipe,  to  fill  it.  In 
this  way  the  entire  flask  and  pipe  will  be 
filled.  This  operation  will  take  place  so 
rapidly  and  with  such  force  as  to  dash  the 
vessel  to  pieces  or  throw  it  from  the  grasp 
of  the  experimenter.  The  violence  with 
which  the  water  rushes  into  the  flask,  is 
due  to  the  suddenness  with  which  the  con- 
densation of  the  steam  takes  place  through- 
out the  entire  vessel. 

The  low-pressure  steam-engine  depends 
upon  this  principle  of  rapid  condensability 
of  vapor  or  steam  for  its  superiority  over  the 
high-pressure  engine. 


Figure  32. — Illustration  of 
the  principle  of  the  low-pressure 
engine. — This  apparatus  consists  of  a  glass 
cylinder,  blown  into  a  bulb  at  its  lower  ex- 
tremity, into  which  is  placed  some  water. 
Into  the  cylinder  is  fitted  a  piston,  with  its  rod  or  handle  projecting 
through  the  cap  or  cover  of  the  cylinder.  In  the  cover  are  two  open- 
ings to  admit  the  egress  and  ingress  of  the  air. 

OPERATION. — If  heat  be  applied  to  the  bulb,  and  steam  generated, 
the  piston  will  be  driven  upward  by  the  steam  with  a  force  equal  to  its 
elasticity,  as  indicated  by  the  arrows  at  W.  If  now  the  cylinder  be 
suddenly  cooled,  by  dipping  it  in  cold  water  or  dashing  cold  water 
upon  it,  the  steam  will  be  condensed,  and  the  downward  pressure  of 
the  air  will  drive  the  piston  down  to  fill  the  vacuum,  with  a  force  equal 


HEAT. 


187 


to  15  Ibs.  to  the  square  inch,  indicated  by  the  arrows  passing  through 
the  cover.     This  operation  can  be  repeated  at  pleasure. 

This  apparatus,  simple  as  it  is,  affords  a  practical  illustration  of  the 
expansion  and  condensation  of  steam,  and  the  atmospheric  pressure, 
which  constitute  the  motive  power  of  the  low-pressure  steam-engine. 

FIG.  32.  FIG.  33. 


334-  Figure  33.— High-pressure  steam.— Under  an  in- 
crease of  pressure  the  boiling  point  rises,  and  the  elastic  force  of  the 
steam  evolved  becomes  correspondingly  greater. 

To  demonstrate  this,  a  spherical  boiler  is  provided,  having  an  in- 
verted mercurial  tube,  A,  with  its  mouth  near  the  bottom  of  the 
boiler;  a  thermometer,  L,  with  its  bulb  near  the  centre;  and  a  stop- 
cock, T.  Sufficient  mercury  is  poured  into  the  boiler  to  supply  the 
tube,  A  (shown  by  the  fine  lines) ;  and  sufficient  water,  W,  to  cover 
the  thermometer  bulb. 

OPERATION. — With  a  spirit-lamp,  set  the  water  to  boiling  with  the 
faucet  open;  and  the  thermometer  will  stand  at  212°,  which  is  the 
boiling  point  under  the  pressure  of  the  air,  or  one  atmosphere  (15  Ibs. 
to  the  square  inch).  If  now  the  faucet  be  closed,  the  steam,  exerting 
its  elastic  force  on  the  surface  of  the  boiling  liquid,  presses  the  mercury 
up  the  tube.  A.  The  height  of  the  mercury  indicates  the  amount  of 


188 


HEAT  AND  STEAM-ENGINE. 


this  pressure ;  and  the  thermometer,  the  corresponding  change  in  the 
temperature  of  the  boiling  point.  When  the  pressure  equals  two 
atmospheres,  the  thermometer  will  show  that  the  boiling  point  has 
risen  to  249°.5  F. 

BOILING.   POINT     OP     WATER    AT     DIFFERENT     ATMOSPHERIC 

PRESSURES. 


NUMBER   OF 

BOILING  POINT 

NUMBER  OF 

BOILING  POINT 

ATMOSPHERES. 

OF  WATER. 

ATMOSPHERES. 

OF  WATER. 

1 

212°  F. 

11 

364°.2  F. 

2 

249.5 

12 

371.1 

3 

273.3 

13 

377.8 

4 

291.2 

14 

384. 

5 

306. 

15 

390. 

6 

318.2 

16 

395. 

7 

329.6 

17 

400.8 

8 

339.5 

18 

405.9 

9 

344.8 

19 

410.8 

10 

356.6 

20 

415.4 

FROST-BEARER.  —  RAIN,     SNOW,    ETC. 

FIG.  34.  335.  Figure  34.— Freezing  by  evap- 

oration— the  cryophorus,  or  frost-bearer. 

— In  this  simple  instrument  water  may  be  frozen 
by  cold  produced  by  its  own  evaporation.  It  con- 
sists of  a  bent  tube,  half  an  inch  or  more  in  diam- 
eter, with  a  bulb,  A  and  F,  at  each  end,  as  repre- 
sented. The  bulb,  F,  is  filled  about  a  third  full 
of  water,  and  the  rest  of  the  space  in  the  instru- 
ment is  full  of  air,  and  only  filled  with  vapor  of 
the  water. 

If  now  the  bulb,  A,  be  immersed  in  a  freezing 
mixture  (309)  of  nitric  acid  and  snow,  the  water 
in  the  distant  bulb,  F,  will  soon  be  frozen.  The 
explanation  is  simple.  The  vapor  in  A  is  rapidly 
condensed,  and  the  rapid  evaporation  of  the  water 
in  F,  to  supply  the  place  of  the  vapor  condensed 
below,  absorbs  or  renders  latent  (303)  so  much  of 
its  own  heat  as  to  reduce  it  to  the  freezing  point. 
The  movement  of  the  vapor  is  indicated  by  the 
arrows. 


Rain  is  the  vapor  of  the  clouds,  or  of 


HEAT. 


189 


the  air,  condensed  and  precipitated  to  the  earth  in  drops.  Rain  is 
generally  occasioned  by  the  union  of  two  or  more  volumes  of  humid 
air,  differing  considerably  in  temperature ;  the  several  portions,  when 
mingled,  being  incapable  of  absorbing  the  same  amount  of  moisture 
that  each  would  retain  if  they  had  not  united.  Hence  the  production 
of  rain  is  the  result  of  the  law,  that  the  capacity  of  air  for  moisture 
decreases  in  a  greater  ratio  than  the  temperature. 

If  the  excess  of  moisture  or  vapor  is  great,  it  falls  as  drops  or  rain ; 
if  it  is  of  slight  amount,  it  appears  as  clouds. 

337.  Snow  is  the  frozen  moisture  that  descends  from  the  atmosphere. 
The  largest  flakes  occur  when  the  atmosphere  is  loaded  with  moist- 
ure, and  the  temperature  is  about  32°  F. ;  as  the  cold  increases,  the 
flakes  become  smaller. 

338.  Hail  is  the  moisture  of  the  air  frozen  into  globules  of  ice. 
Hail-stones  are  generally  pear-shaped ;  and  formed  of  alternate  layers 
of  ice  and  snow,  around  a  white,  snowy  nucleus. 

339.  Figure    35.— Rain   gauge.— This  is  an  instrument  de- 
signed to  measure  the  quantity  of  rain  which  falls  at  any  given  time 
and  place.     It  may  be  made  of  copper  or  zinc,  FIG.  35. 

and  in  the  form  represented. 

For  convenience,  a  communicating  glass  tube, 
L,  is  arranged  outside  of  the  receiving  vessel, 
and  provided  with  a  graduated  scale,  as  shown. 
The  faucet  is  provided  for  drawing  off  the  water. 
If  the  funnel  at  the  top  is  twice  the  size  of  the 
cylinder,  N,  then  an  inch  on  the  scale  would  in- 
dicate half  an  inch  in  the  gauge. 

340.  Distribution  of  rain. — As  a  gen- 
eral rule,  it  may  be  stated  that  the  higher  the 
average  temperature  of  a  country,  the  greater 
will  be  the  amount  of  rain  that  falls.     Local 
causes,  however,  produce  remarkable  departures 
from  this  rule. 

In  Egypt  it  scarcely  ever  rains.  Along  the 
coast  of  Peru,  for  a  long  distance,  it  never  rains. 
No  rain  ever  falls  on  some  portions  of  the  coast 
of  Africa,  while  in  Guiana  it  rains  during  a 
great  part  of  the  year,  as  also  at  the  Straits  of  Ma- 
gellan, and  in  the  Islands  of  Chiloe  (S.  lat.  43°). 


1'JO 


HEAT  AND  STEAM-ENGINE. 


Days  °f  rain. — The  rainy  days  are  more  numerous  in  high 
than  in  low  latitudes,  as  is  seen  in  the  following  table,  although  the 
annual  amount  of  rain  which  falls  is  smaller. 


North  Latitude. 

From  12°  to  43° . 
"  43°  "  46°. 
«  46°  "  50°. 
"  50°  "  60° 


Mean  Annual  Number  of  Rainy  Days 

78. 

103. 

134. 

,  161. 


Annual  depth  of  rain. — The  greatest  annual  depth  of 
rain  occurs  at  San  Luis,  Maranham,  280  inches ;  the  next  in  order  are 
Vera  Cruz,  278 ;  Grenada,  126 ;  Cape  Frai^ois,  120 ;  Calcutta,  81 ; 
Rome,  39;  London,  25;  Uttenburg,  12.5;  Hanover,  N.  H.,  38;  New 
York  State,  36  ;  Ohio,  42  ;  Missouri,  38. 


FIG.  36. 


343.  Figure  36. — Hygrometer,  or  moisture  measurer. 

— The  use  of  this  instrument  is  to  show  the  state  of  moisture  in  the  at- 
mosphere. It  consists  of  two  glass 
bulbs,  connected  by  a  glass  tube 
twice  bent,  as  shown.  The  bulb, 
A,  contains  a  small  quantity  of 
ether,  by  the  boiling  of  which  the 
air  has  been  expelled  from  the  in- 
strument. It  contains  a  small 
thermometer  with  its  bulb  in  the 
ether.  The  bulb,  T,  is  covered 
with  muslin.  Upon  the  support- 
ing column  is  attached  another 
thermometer. 

OPERATION.  —  Let  fall  a  few 
drops  of  ether  upon  the  bulb,  T, 
and  its  evaporation  will  reduce  the 
temperature  of  the  bulb,  A,  by 
causing  the  ether  within  to  evap- 
orate to  supply  vapor  to  take  the 
place  of  the  condensed  vapor  in  T 
(see  Fig.  34).  When  the  temper- 
ature, of  the  bulb,  A,  is  thus  suf- 
ficiently reduced,  the  moisture  of 
the  air  will  begin  to  accumulate 
upon  the  outside.  This  is  called  the  dew-point,  the  temperature  of 
which  is  shown  by  the  thermometer  within.  The  temperature  of  the 
dew-point  varies  with  the  amount  of  moisture  in  the  air  (328\  The 


UK  AT. 


191 


FIG.  37. 


greater  the  amount  of  moisture,  the  nearer  the  dew-point  is  to  the 
temperature  of  the  air ;  hence  the  difference  between  the  two  ther- 
mometers will  indicate  the  relative  humidity  of  the  atmosphere.  The 
drier  the  air  the  greater  is  this  difference. 

344-  Fi&ure  37.— Combustion  and  structure  of  flame.— 

The  diagram  illustrates  some  of  the  principles  involved  in  the  burning 
of  a  jet  of  illuminating  (hydro-carbon)  gas. 

There  are  four  simple  elements  or  kinds  of  matter,  which,  combined 
in  various  proportions,  make  up  the 
great  bulk  of  all  organic  bodies,  both 
animal  and  vegetable.  These  are  oxygen, 
hydrogen,  carbon,  and  nitrogen,  and  may 
be  called  the  four  organic  elements. 

Of  these  four  elements,  carbon  and 
hydrogen  are  those  which  impart  to  or- 
ganic compounds  the  property  of  com- 
bustibility ;  the  oxygen  and  nitrogen  in 
them  only  regulating  the  intensity  with 
which  they  burn.  The  compounds  em- 
ployed as  sources  of  heat  and  light,  con- 
tain organized  hydrogen  and  carbon. 
Carbon  and  hydrogen  burned  separately, 
give  rise  each  to  a  large  amount  of  heat; 
but  they  exist  in  different  forms,  and 
burn  in  different  ways.  Carbon  is  a 
solid,  and  remains  so  during  combustion. 
Hydrogen  is  a  gas,  and  burns  as  a  gas ; 
and  if  set  free,  it  diffuses  into  the  air, 
and  thus  burns  while  in  motion,  giving  rise  to  flame,  and  heating  the 
particles  of  carbon  to  a  white  heat,  which  is  the  principal  source  of  the 
luminosity  of  flame. 

The  best  illuminators,  therefore,  are  pure  hydro-carbons,  composed 
of  an  equal  number  of  atoms,  or  equivalents,  of  each. 

The  diagram  represents  a  section  of  a  gas-jet  flame.  The  burning 
of  other  substances,  as  tallow,  liquid  oils,  etc.,  involve  the  same  prin- 
ciples ;  for  these  must  be,  and  are,  converted  into  gas  before  they  are 
burned. 

The  gas-jet  or  candle-flame  is  not  a  solid  mass  of  fire,  but  a  hollow 
shell  of  light,  and  is  dark  within,  as  shown  by  the  diagram ;  H  being 
the  dark  space.  The  dark  chamber  within  is  filled  with  the  combus- 
tible gas. 

Illuminating  gases  being  composed,  in  part,  of  hydrogen,  they  are 


192  HEAT  AND  8TEAM-ENGINE. 

lighter  than  the  air,  and  tend  to  rise,  which,  with  the  heat  of  combus- 
tion, produce  an  upward  current,  causing  the  flame  to  ascend,  and 
giving  it  a  conical  or  pointed  form. 

The  air  being  composed  of  1  part  of  oxygen  to  4  of  nitrogen  (124), 
aifords  just  enough  free  oxygen  to  support  safe  combustion  (211). 
The  free  oxygen  of  the  air  chemically  combining  with  the  hydrogen 
and  carbon,  constitutes  the  combustion ;  which  results  in  water  and 
carbonic  acid  gas. 

The  hydrogen  is  oxidized  first,  producing  intense  heat,  which  sets 
free  the  minute  particles  of  carbon,  and  heats  them  to  whiteness,  giving 
rise  to  a  vivid  white  light.  But  if  the  particles  of  carbon  are  sufficiently 
heated,  they  become  oxidized  and  lose  their  luminosity  or  whiteness. 

Now,  by  observing  an  actual  gas-jet,  it  will  be  noticed  that  at  the 
lower  part  of  the  flame,  where  the  arrows  (in  the  diagram)  point 
toward  the  jet  (up  as  far  as  AA),  the  flame  is  dark,  or  nearly  so.  This 
is  because  the  abundant  supply  of  air  (represented  by  the  same  arrows) 
affords  sufficient  oxygen  to  cause  complete  combustion  at  this  point 
of  the  flame ;  and  the  outer  portion  of  the  flame  (shown  by  the  dots)  is 
also  bluish  instead  of  ivhite,  for  the  reason  that,  coming  in  contact  with 
the  air,  it  receives  sufficient  oxygen  to  render  the  combustion  more 
complete  than  it  is  a  little  deeper  in  the  flame,  but  not  so  complete 
as  at  the  bottom ;  hence  the  surface  is  not  as  dark  as  the  bottom. 
That  part  of  the  flame  between  the  dark  chamber,  H,  within,  and  the 
bluish  portion  without,  is  the  principal  illuminating  part  of  the  blaze. 
In  this  part  of  the  flame  the  amount  of  oxygen  of  the  air  is  only  suf- 
ficient to  burn  the  hydrogen,  the  heat  of  which  only  whitens,  but  does 
not  burn  the  carbon;  while  the  dark  chamber  within  is  nothing  but 
the  hydro-carbon  gas  itself;  no  oxygen  of  the  air  coming  in  contact 
with  it  to  set  up  combustion. 

The  arrows  above  AA  show  the  upward  currents  of  the  air  and 
gases  after  combustion. 

STEAM  -ENGINES. 

345.  Origin  of  the  steam-engine. — A  steam-engine  is  any 
contrivance  for  converting  heat  into  mechanical  energy  through  the 
medium  of  water. 

The  first  rudiments  of  knowledge  of  steam  as  a  motor  date  back  of 
modern  times.  As  early  as  130  years  before  the  Christian  era,  Heiro 
describes,  among  other  curious  contrivances,  the  eolipile. 

346.  Figure  38. — The  eolipile. — A  form  of  this  apparatus  is 
shown  by  the  figure.     It  consists  of  a  metallic  globular  boiler,  A,  pro- 
vided with  two  hollow  arms,  holding  a  hollow  cross-bar,  T,  from  which 


STEAM-ENGINE. 


193 


FIG.  38. 


project  two  other  arms,  at  the  ends  of 
which,  on  opposite  sides,  are  openings  for 
steam  to  escape.  In  the  boiler  is  water, 
which  is  converted  to  steam  by  heat.  The 
steam  passes  into  the  cross-bar,  where  it 
comes  in  contact  with  the  side-arms  of  the 
boiler,  and  escapes  at  the  openings,  as 
shown.  The  escaping  steam  recoils  on  the 
atmosphere,  and  revolves  the  cross-bar 
with  great  rapidity.  The  mechanical  prin- 
ciple involved  is  the  same  as  that  of  Barker's 
Mill  (163),  the  pressure  of  steam  taking 
the  place  of  the  pressure  of  water. 


347 .  Improvements   in    steam- 
engines. — It  cannot  be  expected  that  in 
a  work  like  this  an  explanation  and  history 
can  be  given  of  the  many  improvements 
which,  from  time  to  time,  have  been  made 
in  the  construction  of  the   steam-engine. 

The  essential  principles,  however,  involved  in  the  application  of  steam 
as  a  motor,  and  the  principal  parts  of  high-pressure  and  low-pressure 
engines,  will  be  understood  by  reference  to  what  has  already  been  said, 
and  to  the  following  five  illustrations  and  their  explanations. 

Whatever  may  have  been  the  construction  of  the  first  contrivance 
that  merited  the  name  of  steam-engine,  or  when,  or  where,  or  by  whom 
invented,  there  was  nothing  that  could  compare  with  the  improvements 
made  by  James  Watt,  in  1769.  What  was  done  -before  was  important 
chiefly  in  leading  the  way  to  his  improvements.  Of  course,  many 
minor  and  some  important  improvements  have  been  made  in  the  con- 
struction of  engines  since  Watt's  time ;  but  no  general  principles  of  im- 
portance have  since  been  discovered. 

348.  Reciprocating   and  rotary  motion  of  engines. — 

There  are  rotary  steam-engines,  but  these  are  not  commonly  employed. 
All  steam-engines  in  general  use  are  reciprocating ;  that  is,  they  are 
provided  with  a  cylinder  and  piston.  The  steam  works  the  piston  back 
and  forth  in  the  cylinder  (as  will  be  seen) ;  and  this  is  called  recipro- 
cating motion  ;  which  is  converted,  by  means  of  a  crank,  into  rotary 
motion.  There  are  many  objections  to  reciprocating  engines  which 
would  be  wholly  obviated  by  rotary  engines,  were  it  possible  to  simplify 
the  construction  of  the  latter,  and  render  them  as  durable  as  the 
former. 

13 


194 


HEAT  AND  STEAM-ENGINE. 


FIG.  39. 


349.  Figure  39.— The  high-pressure  engine.— The  high- 
pressure  engine  is  employed  for  railroad  locomotives  and  small  steam- 
boats,   especially    harbor    tug- 
boats, and  all,  or  nearly  all,  sta- 
tionary engines. 

In  this  machine,  the  escape 
steam  is  driven  out  against  the 
pressure  of  the  atmosphere ;  and 
instead  of  advantage  being  taken 
of  the  condensibility  (332  and 
333)  of  the  steam  and  atmo- 
spheric pressure,  the  steam 
must  exert  a  force  of  15  Ibs.  to 
the  square  inch,  to  overcome 
the  atmospheric  pressure,  before 
it  becomes  effective  for  use ;  yet 
the  lightness,  simplicity,  com- 
pactness, and  low  cost  of  the 
high-pressure  engine  render  it 
available,  notwithstanding  its 
uneconomical  use  of  steam,  in 
many  places  where  the  low- 
pressure  or  condensing  enginfe 
could  not  be  employed. 

In  the  figure,  F  is  the  piston, 
fitted  steam-tight  to  the  cylinder  which  surrounds  it ;  S,  the  steam-pipe 
that  conveys  the  steam  from  the  boiler  into  the  steam-chest,  A,  which 
communicates  with  the  interior  of  the  cylinder  at  the  top  and  bottom. 
N"  is  the  discharge  or  ejection  pipe,  which  communicates  with  the 
opening,  L,  in  the  bottom  of  the  steam-chest ;  T,  the  cut-off  or  slide- 
valve,  which  is  fastened  to  and  operated  by  the  rod,  H,  passing  through 
the  end  of  the  steam-chest. 

OPERATION. — As  the  piston  stands  (in  the  figure),  the  steam  from  the 
steam-pipe  passes  into  the  steam-chest  and  through  it  into  the  cylinder 
above  the  piston,  as  shown  by  the  arrow,  which  presses  it  down  to  the 
bottom  of  the  cylinder,  while  the  steam  below  the  piston  is  driven  out 
through  the  lower  passage  into  the  opening,  L,  as  shown  by  the  other 
arrow,  which  communicates  with  the  ejection -pipe. 

If  now  the  sliding- valve,  T,  be  moved  up,  by  means  of  its  rod,  H, 
until  its  upper  bearing  (now  between  the  arrows)  passes  the  upper 
passage,  it  will  connect  the  upper  passage  with  the  discharge-pipe,  as 
now  the  lower  passage  is,  while  it  will  open  a  communication  between 


STEAM-ENGINE. 


195 


the  steam-chest  and  the  lower  end  of  the  cylinder,  as  now  there  is 
between  it  and  the  upper  end. 

The  sliding-valve  being  thus  moved,  the  steam  will  now  force  the 
piston  back.  By  alternately  shifting  the  valve,  the  steam  is  alternately 
admitted  and  discharged  at  the  opposite  ends  of  the  cylinder,  above 
and  below  the  piston. 

The  supply  of  steam,  and  the  connection  of  the  piston-rod  to  the 
resistance  to  be  overcome,  and  the  means  of  working  the  valves,  will  be 
shown  hereafter. 

350-  Figure  40. — The  eccentric. — Its  importance.— As  at 

every  stroke  of  the  piston  the  valves  must  be  shifted  or  reversed,  it  is 

important  that  the  engine  it- 

self  be  made  to  perform  this 

operation ;  as  it  is  only  by  this 

means  that  the   steam-engine 

can  be  made  automatic  in  its 

operations,  and  without  which 

it  would  possess  but  a  limited 

usefulness. 

To  accomplish  this  indispen- 
sable part  of  the  operation  is 
the  object  of  the  eccentric, 
which  consists  of  a  wheel, 
keyed  on  the  main  shaft  of 
the  engine ;  the  hole,  through 
which  the  shaft  passes,  being 
made,  instead  of  at  the  centre, 
at  one  side  of  the  centre,  as 
at  S.  Around  the  periphery 
of  this  wheel,  in  a  groove,  is 
clasped  an  iron  band,  bolted 
together,  as  at  LL.  E  is  sim- 
ply an  opening  in  the  wheel, 
made  to  lighten  it  and  save 
metal.  From  this  band  extends  a  rod,  A,  called  the  eccentric-rod, 
which  is  welded  to,  and  is  a  part  of,  the  valve-rod,  as  H  in  the  last 
diagram. 

OPERATION. — As  the  shaft,  S,  revolves,  the  wheel  turns  in  the  band. 
Suppose  the  shaft  to  be  revolved  half  a  turn,  which  is  accomplished  by 
a  single  stroke  of  the  piston  of  the  engine ;  then  the  wheel  will  take 
the  position  of  the  dotted  circle,  which  will  lift  the  eccentric-rod.  A,  a 
distance  equal  to  the  distance  from  the  upper  point  of  the  circumfer- 


196 


HEAT  AND  STEAM-ENGINE. 


ence  of  the  wheel,  in  its  former  position,  to  the  upper  point  of  the 
dotted  circle ;  which  is  sufficient  to  shift  or  reverse  the  valves.  Thus, 
at  every  stroke  of  the  piston  and  every  half  revolution  of  the  shaft,  the 
valves  are  reversed  by  the  eccentric. 

351.  Figure  41. — Steam-boiler  and  operation  of  steam- 
valves. — Boilers  are  made  of  heavy  plate-iron,  firmly  riveted  together, 
provided  with  suitable  means  for  heating  them,  to  produce  steam. 
They  are  partly  filled  with  water,  the  balance  of  the  space  being  filled 
with  steam,  which  supplies  the  engine. 

The  object  of  the  three  faucets,  1,  2,  3,  is  to  enable  the  engineer  to 
ascertain,  at  any  moment,  the  amount  of  water  there  may  be  in  the 

FIG.  41. 


boiler.  As  the  water,  W,  should  be  kept  about  on  a  level  with  the 
middle  faucet,  if  the  upper  faucet  be  opened  there  should  escape  from 
it  only  steam  ;  but  if  water  should  escape,  it  shows  there  is  too  much 
water  in  the  boiler.  If  mingled  water  and  steam  escape  from  the 
middle  faucet  it  shows  the  boiler  is  properly  filled ;  but  if  only  steam 
escapes,  it  shows  the  water  is  low ;  and  if  steam  issues  from  the  lower 
faucet  it  indicates  the  water  is  dangerously  low.  The  level  of  the  water 
in  the  boiler  is  also  shown  by  communicating  glass  tubes  arranged  on 
the  outside  of  the  boiler. 


STEAM-ENGINE. 


197 


V  is  an  outward-acting  safety-valve,  to  allow  the  steam  to  escape 
when  its  pressure  has  reached  the  point  beyond  which  it  is  unsafe.  The 
ball  and  lever,  and  the  size  of  the  valve,  enable  the  engineer  to  deter- 
mine and  fix  the  limit  of  pressure  at  different  times. 

The  inward-working  safety-valve,  L,  is  to  prevent  the  boiler  from 
being  collapsed  by  the  atmospheric  pressure  without,  in  the  event  of 
the  steam  becoming  condensed  within. 

A  is  the  throttle-valve,  which  governs  the  supply  or  flow  of  steam  to 
the  cylinder;  P,  the  piston;  E,  the  piston-rod,  passing  through  a 
stuffing-box  at  the  head  of  the  cylinder.  The  action  of  the  steam  on 
the  piston  is  controlled  by  the  four  valves,  T,  E,  F;  and  H.  By  tracing 
the  arrows  (which  represent  steam)  out  of  the  boiler,  it  will  be  seen 
that  T  is  open  and  F  closed,  so  that  the  piston  is  being  driven  down 
by  the  pressure  of  the  steam  on  its  upper  surface,  as  indicated  by  the 
two  short  arrows;  while  the  steam  below  the  piston  is  being  driven 
through  the  valve  H,  and  kept  from  escaping  above  the  piston  by  the 
closed  valve  E,  as  the  arrows  indicate. 

By  reversing  the  valves,  the  piston  will  be  forced  in  the  opposite 
direction  ;  and  so  on. 

There  are  many  ways  of  arranging  the  valves  in  different  engines ; 
but,  however  constructed  and  arranged,  the  principle  of  reversing  them 
is  the  same. 

852.  Condensation  in  steam-engines.  —  Referring  to  the 
above  diagram  (Fig.  41),  if  by  any  means  the  valve,  H,  could  be  closed, 
and  the  steam,  below  the  piston,  suddenly  con-  FIGi  43. 

densed,  the  upward  pressure  or  resistance  of  the 
atmosphere  (15  Ibs.  to  the  square  inch)  would 
be  removed ;  which  would  be  equivalent  to  in- 
creasing the  elastic  force  of  the  steam  15  Ibs. 
to  the  square  inch.  This,  however,  is  accom- 
plished by  the  condensing  or  low-pressure  en- 
gine (Fig.  43). 

The  advantage  of  the  atmospheric  pressure 
on  a  piston  of  tw o  feet  diameter  is  nearly  three 
and  a  half  tons. 

353.  Figure  42.  —  Stuffing-boxes.— 

The  object  of  these  is  to  furnish  a  working 
steam-tight  joint  or  fitting  between  the  piston- 
rod  and  cylinder-head ;  as  also  to  provide  such  a 
joint  or  packing  in  all  cases  where  gases,  va- 
pors, or  fluids  are  to  be  confined  against  pressure. 


198  HEAT  AND  STEAM-ENGINE. 

Let  H  be  a  section  of  the  piston-head,  with  a  hollow,  dish-like  pro- 
jection, L,  on  its  upper  surface,  filled  with  cotton,  hemp,  or  other  fibrous 
substance,  shown  by  the  small  dots ;  over  which  is  placed  a  downward, 
projecting  disk  or  collar,  which  crowds  upon  the  fibrous  substance,  and 
drives  or  presses  it  against  the  rod,  A,  as  represented  by  the  direction 
of  the  arrows.  The  force  with  which  the  hemp  or  other  material  is 
pressed  against  the  piston-rod  is  regulated  by  the  two  bolts  and 
nuts,  TT. 

354'  Figure  43. — The  low-pressure  or  condensing  en- 
gine (see  frontispiece). — The  low-pressure  engine  is  employed  on  all 
large  steamboats,  and  in  situations  where  economy  of  fuel  and  the  great- 
est mechanical  eifects  from  it  are  the  principal  considerations. 

Owing  to  the  nearly  perfect  vacuum  obtained  by  the  condenser  and 
air-pump,  about  14  Ibs.  to  the  square  inch  of  the  atmospheric  pressure 
is  removed,  which  adds  so  much  to  the  mechanical  energy  of  the  steam 
(352).  Hence,  with  a  pressure  of  only  10  Ibs.  of  steam  to  the  inch,  a 
mechanical  force  of  24  Ibs.  to  the  inch  is  obtained;  which  shows  the 
propriety  of  the  term  low-pressure  engine. 

S  is  the  steam-pipe,  which  conveys  the  steam  from  the  boiler  to  the 
engine ;  just  at  the  opening  of  which  is  seen  the  throttle-valve,  which 
controls  the  flow  of  steam.  B  is  a  double-acting  cylinder;  N,  the 
valve-rod,  provided  with  adjustable  arms,  and  connected  to  the  valves 
in  the  steam-chests ;  Y,  a  right-angular  bar  or  lever,  which  works  the 
valve-rod ;  F,  the  eccentric-rod,  that  operates  the  valve-lever ;  U,  the 
fly-wheel,  on  the  shaft  of  which  is  the  eccentric;  D,  the  pitman,  which 
connects  the  working-beam  above  with  the  crank  of  the  shaft ;  Z,  a  beam 
on  which  rests  the  bearing  of  the  working-beam ;  HIJK,  the  parallelo- 
gram which  produces  parallel  motion  and  vertical  action  of  the  piston- 
rod  ;  JR,  the  radius-rod.  The  beam,  on  which  the  working-beam  rests, 
is  supported  by  an  iron  column  in  the  centre ;  0,  a  triangular  bar  to 
communicate  the  action  of  the  governor  to  the  throttle-valve,  S,  in  the 
steam-pipe.  EE  is  the  cold-water  cistern,  in  which  is  contained  and 
immersed  the  condenser,  L,  and  air-pump,  T ;  V,  the  rod  which  opens 
the  passage  for  cold  water  to  pass  from  the  cistern  into  the  condenser : 
P,  the  pump  which  draws  the  hot  water  from  the  hot- water  chamber 
(above  A)  in  the  cistern,  and  sends  it  to  the  boiler ;  W,  the  pump 
which  supplies  the  cistern  with  cold  water  from  the  well  or  other 
source. 

Operation. — By  means  of  the  eccentric  on  the  shaft,  the  eccentric- 
rod,  F,  and  right-angle  lever,  Y,  and  arms  on  the  valve-rod,  N,  the 
valves  in  the  steam-chests  (at  the  upper  and  lower  ends  of  the  cylinder) 


STEAM-ENGINE.  199 

are  alternately  reversed,  as  the  piston  is  moved  up  and  down.  The 
exhaust-steam  is  met  in  the  condenser,  L,  by  a  jet  of  cold  water  from 
the  cistern,  EE,  which  condenses  the  residuum  steam  and  produces  a 
vacuum  (332)  in  the  cylinder,  on  the  side  of  the  piston  opposite  to  the 
pressure  of  the  steam.  The  condenser,  L,  is  kept  exhausted  of  water 
and  air  by  means  of  the  air-pump,  T.  The  outward-working  valve  at 
the  bottom  of  the  condenser,  and  the  upward-working  valve  in  the 
plunger  of  the  air-pump,  will  be  readily  understood. 

As  the  piston  of  the  air-pump,  T,  descends,  the  valve  in  the  hot- 
water  chamber  closes  to  prevent  the  hot  water  from  returning  to  the 
air-pump;  and  the  valve  between  the  air-pump  and  condenser  closes 
to  prevent  the  contents  of  the  air-pump  from  returning  to  the  con- 
denser. 

The  air-pump  discharges  its  water  into  the  hot-water  cistern  above  A, 
from  which  it  is  drawn  by  the  pump  P,  and,  to  economize  heat,  is  sent 
through  the  dotted  pipe  in  the  brick- work  to  contribute  toward  feeding 
the  boiler. 

The  cistern,  EE,  is  kept  supplied  with  cold  water  by  the  pump,  W. 

The  governor. — The  throttle- valve,  S,  which  admits  steam  to  the 
piston,  is  controlled  by  the  governor;  which  consists  of  two  heavy  iron 
balls,  suspended  on  arms,  as  shown.  These  arms  are  pivoted  on  a 
central  spindle,  which  is  made  to  revolve  by  means  of  a  cord  or  belt 
(shown  by  the  dotted  lines  passing  over  the  two  friction  rollers)  con- 
necting the  main  shaft  with  a  pulley  at  the  bottom  of  the  spindle. 
The  balls  are  swung  out  from  the  spindle  by  centrifugal  force.  If  the 
engine  is  running  too  fast,  the  balls,  by  rising  higher,  will  draw  the 
sliding  collar  or  sleeve,  M,  down  on  the  spindle,  which  throws  the  per- 
pendicular arm  of  the  right-angle  bar  to  the  right ;  which,  being  con- 
nected to  the  arm  of  the  throttle- valve  by  an  iron  rod,  partially  closes 
the  valve,  and  thus  diminishes  the  supply  of  steam.  When,  by  this 
means,  the  speed  of  the  engine  is  sufficiently  reduced,  the  balls,  being 
depressed  by  gravity,  reverse  the  motion  of  the  throttle-valve  and  again 
let  on  more  steam. 

The  arm  of  the  throttle-valve  is  provided  with  a  series  of  holes  to 
regulate  its  opening,  to  adjust  the  supply  of  steam  to  the  amount  of 
work  to  be  performed  by  the  engine  at  different  times. 

The  fly-wheel.— As  the  crank  revolves,  there  are  two  points  or 
positions  where  it  is  said  to  be  on  "  the  dead-centre."  When  the  pit- 
man, D,  works  vertically,  the  dead-centres  are  at  the  highest  and  low- 
est points  reached  by  the  crank-pin.  If  the  engine  is  at  rest  and  the 
crank  is  in  either  of  these  two  positions,  the  fly-wheel  must  be  revolved 


N£00  HEAT  AND  STEAM-ENGINE. 

a  little,  by  hand,  before  the  steam  can  move  it,  as  all  its  force  would 
act  in  a  straight  line  running  through  the  centre  of  the  shaft,  which 
would  have  no  tendency  to  revolve  the  wheel. 

There  are  also  two  points,  in  the  revolution  of  the  crank  where  the 
power  of  the  steam  is  the  most  effective  in  revolving  the  wheel ;  and 
these  are  at  right  angles — or  90° — from  the  dead-centre  points.  From 
2  (one  of  the  dead-centres)  the  power  obtains  a  stronger  and  stronger 
hold  on  the  crank  until  it  reaches  the  position  of  1  (the  piston  being  at 
u  half  stroke") ;  then  its  hold  grows  less  and  less  until  it  reaches  the 
opposite  dead-centre  point,  where,  again,  the  power  becomes  wholly 
ineffective  to  revolve  the  crank ;  and  so  on. 

This  will  explain  one  of  the  reasons  why  a  heavy  fly-wheel  is  indis- 
pensable with  a  single-crank  engine.  As  there  are  but  two  points  in 
each  revolution  of  the  crank  where  the  power  of  the  steam  can  exert 
its  entire  force,  it  becomes  necessary  to  employ  the  fly-wheel,  to  equal- 
ize the  power  in  its  application  to  the  resistance,  and  give  the  engine  a 
steady  and  uniform  motion. 

In  steamboats,  however,  the  weight  of  the  crank  and  wheels,  together 
with  the  motion  of  the  boat,  act  as  a  substitute  for  a  fly-wheel.  The 
wheels  of  railroad  locomotives  and  the  momentum  of  the  entire  ma- 
chines act  as  fly-wheels. 

The  unequal  resistance  offered  to  the  engine  from  one  moment  to 
another,  as  in  rolling-mills,  etc.,  also  makes  it  necessary  to  employ  the 
fly-wheel,  as  otherwise  the  engine  at  one  moment  would  be  resisted 
beyond  its  power,  while  at  another  it  would  have  no  resistance. 

Parallel  motion. — The  object  of  the  parallelogram,  HIJK,  is 
to  produce  an  upright  or  vertical  motion  to  the  piston-rod  without 
slides;  though  slides  are  now  usually  employed.  The  radius,  JR, 
being  fixed  at  R  to  the  beam,  determines  the  arc  of  the  circle  through 
which  the  point,  J,  moves,  which  causes  the  point,  K,  or  head  of  the 
piston-rod,  to  travel  in  a  straight  perpendicular  line. 


OPTICS.  201 


CHAPTEE    X. 

(CHART  NO.  5.) 

OPTICS. 
General  Properties  of  Light. 

355.  Definitions. — Optics. — Optics  is  that  branch  of  Physics 
which  treats  of  the  nature  and  properties  of  light. 

Light  is  that  mysterious  agent  which,  acting  upon  the  organs  of 
vision,  produces  the  sensation  of  sigfrt.  Light  unfolds  to  us  the  beauties 
of  nature,  and  brings  us  into  convenient  and  pleasing  relation  with 
surrounding  objects. 

356.  Nature  of  light.— Theories. — Two  theories  have  been 
advanced  to  account  for  the  phenomena  of  light;  the  Corpuscular  or 
Emission  Theory,  and  the  Wave  or  Undulatory  Theory. 

According  to  the  corpuscular  or  emission  theory,  light  consists  of 
infinitely  small  particles  of  matter  shot  forth  from  burning  or  luminous 
bodies,  with  immense  velocity,  which,  falling  upon  the  retina  of  the 
eye,  produce  the  sensation  of  sight. 

According  to  the  wave  or  undulatory  theory,  light  consists  of  waves 
or  undulations  in  the  ethereal  medium,  caused  by  luminous  bodies 
acting  upon  it ;  and  that  these  waves  dash  upon  the  retina  of  the  eye, 
producing  sight ;  same  as  waves  of  air  fall  upon  the  ear,  and  produce 
the  sensation  of  sound.  By  this  theory  no  matter  whatever  is  thrown 
off  from  the  luminous  body.  The  ethereal  medium  which  the  lumi- 
nous body  acts  upon  or  sets  in  motion,  is  supposed  to  be  universally 
existent,  impalpable  or  imponderable,  and  extremely  elastic. 

Though  there  has  been  a  great  diversity  of  opinion  respecting  the 
nature  of  light,  the  undulatory  theory  is  now  most  generally  received, 
as  it  more  satisfactorily  accounts  for  the  various  phenomena ;  though 
it  is  difficult  to  explain  all  the  phenomena  of  light  even  on  this  theory. 
No  theory  of  light  is  entirely  satisfactory. 

357.  Sources  of  light. — Bodies  which  give  out  or  emit  light 
are  called  luminous  bodies.    The  sources  of  light  are  the  sun,  stars, 
heat,  chemical  combinations,  phosphorescence,  and  electricity. 


202  OPTICS. 

Nothing  is  known  of  the  cause  of  the  light  emitted  by  the  sun  and 
stars.  It  is  known,  however,  that  bodies  become  luminous  at  a  high 
temperature,  and  the  greater  the  intensity  of  the  heat,  the  more  vividly 
they  shine. 

Artificial  light,  as  of  candles,  lamps,  gas,  etc.,  is  due  to  combustion 
of  substances  containing  carbon  and  hydrogen.  See  structure  of  flame, 
344.  The  chemical  action  in  such  cases  disengages  more  or  less  in- 
tense heat,  as  the  burning  body  becomes  luminous. 

Phosphorescence  is  a  pale  light  emitted  in  the  dark  by  certain  sub- 
stances which  do  not  appear  to  emit  any  sensible  heat,  a  beautiful 
specimen  of  which  may  be  seen  by  looking  at  cold  boiled  lobster  in  the 
dark.  It  has  been  observed  in  animals,  vegetables,  and  minerals;  the 
glow-worm  and  fire-fly  emit  it.  Under  certain  conditions  of  the  air 
and  water,  magnificent  displays  of  it  may  be  seen  in  the  track  of  a 
steamboat  in  salt-water.  The  cause  of  phosphorescence  is  not  known, 
but  in  some  cases  it  appears  to  depend  upon  electricity. 

Electricity  is  a  source  of  light  so  intense  that  its  brightness  is  equal, 
in  some  cases,  from  one-fifth  to  one-fourth  that  of  the  sun. 

358.  Similarity  of  light  and  heat. — There  are  several  reasons 
for  supposing  that  the  thermal,  luminous,  and  chemical  rays  (420)  of 
sunbeams  are  one  and  the  same ;  among  which  are : 

1.  Light  and  radiant  heat  are  reflected,  refracted,  dispersed,  and 
polarized  in  the  same  way. 

2.  The  dark  lines  in  the  solar  spectrum  are  devoid  of  heat. 

3.  Similar  dark  and  cold  lines  exist  in  the  obscure  part  of  the  spec- 
trum beyond  the  red  end  where  the  heat  is  most  intense. 

4.  There  is  an  absence  of  both  heat  and  chemical  action  in  the  dark 
lines  found  in  the  luminous  part  of  the  spectrum. 

3 59.  Relation  of  different  bodies  to  light.— Bodies,  as  re- 
lated to  light,  are  either  luminous,  non-luminous,  transparent,  translu- 
cent, or  opaque. 

Luminous  bodies  are  those  in  which  light  originates,  as  the  sun  and 
burning  bodies. 

Non-luminous  bodies  are  those  which  are  seen  by  reflected  light,  as 
the  earth,  the  moon,  a  rock,  a  house,  etc. 

Transparent  bodies,  also  said  to  be  diaphanous  (meaning  to  shine 
through),  permit  light  to  pass  through  them  freely,  as  glass,  water, 
and  air. 

Translucent  bodies  are  such  as  imperfectly  transmit  light,  but  not 
sufficiently  to  show  the  outlines  of  objects,  as  ground  glass,  oiled  paper, 
thin  porcelain,  etc. 


OPTICS.  203 

Opaque  bodies  are  those  which  do  not  ordinarily  allow  any  light  to 
pass  through  them,  as  wood,  iron,  etc. ;  though  thin  leaves  of  some 
metals  partially  transmit  some  colors ;  thin  leaves  of  gold,  for  instance, 
transmit  a  beautiful  violet-green  light. 

360.  A  medium. — Propagation  of  light  in  a  homoge- 
neous medium. — A  medium  is  luminiferous  when  it  transmits  light, 
and  it  is  homogeneous  when  its  composition  and  density  are -the  same 
in  all  its  parts.     All  space  is  supposed  to  be  pervaded  by  such  a  me- 
dium, called  luminiferous  ether.      In  a  homogeneous  medium  light 
always  moves  in  straight  lines,  as  may  be  shown  by  admitting  light 
into  a  dark  chamber  by  a  very  small  opening,  which  renders  the  course 
of  the  light  visible,  by  its  illumination  of  the  tine  particles  of  dust 
always  floating  in  the  air.     Media,  such  as  water,  air,  and  glass,  are 
pervaded  among  their  particles  by  the  luminiferous  ether,  but  not 
always  in  such  a  state  as  to  permit  the  transmission  of  light. 

361.  Absorption  of  light.— No  body  is  perfectly  transparent; 
all  intercept  or  absorb  a  portion  of  light.     Some  media,  which  in  thin 
layers  are  transparent,  intercept  or  absorb  most  of  the  light,  if  they  be 
very  thick.     Opaque  bodies  absorb  all  of  the  light  falling  upon  them 
which  is  not  reflected  (see  Opaque  Bodies,  359).     The  cause  of  absorp- 
tion is  some  peculiarity  of  molecular  constitution,  which  breaks  up 
and  neutralizes  the  waves  of  light  that  enter  them. 

Though  the  air  appears  perfectly  transparent,  much  of  the  light 
of  the  sun  is  absorbed  by  it  in  reaching  the  earth,  FIG.  1. 

shown  by  the  greater  brilliancy  of  the  stars  when 
viewed  from  high  mountain-tops.  In  fact,  so  great 
is  the  clearness  of  vision  in  the  higher  regions  of 
the  atmosphere,  that  it  becomes  exceedingly  diffi- 
cult to  judge  of  distances. 


362.  Figure  1.  — Rays,  pencils,  and 
beams  of  light. — A  single  line  of  light  is 
called  a  ray. 

A  pencil  of  light  is  a  collection  of  rays  diverging 
from  or  converging  to  a  common  point,  as  shown 
in  the  diagram.  Hence  there  are  converging  and 
diverging  rays. 

A  beam  of  light  is  a  small  collection  of  parallel  rays,  such  as 
would  pass  through  a  hole  in  a  shutter,  from  a  distant  body,  as  the 
sun. 


204 


OPTICS. 


FIG.  2. 


363.  Figure  2.— Visible  bodies  emit  light  from  every 
point,  and  in  every  direction,  the  rays  diverging  from  each 

point  in  straight  lines.  The 
two  eyes,  in  the  figure,  receive 
rays  of  light  from  the  same 
three  points  of  the  arrow,  but 
not  the  same  rays.  So  each 
and  every  point  of  the  object 
emits  light  in  every  direc- 
tion, but  the  eye  in  any  one 
position  only  takes  in  the 
rays  that  come  in  its  direc- 
tion. There  are  110  vacant 
spaces  among  the  rays,  as  at 
B,  in  the  diagram,  but  the 
entire  space  is  filled  with 
rays  crossing  each  other  at 
every  point. 

364-  Properties  of  light. — Light  falling  upon  any  substance  is 
either  absorbed,  dispersed,  reflected,  or  refracted. 

Absorption. — Light  falling  upon  a  black  substance  partly  disappears, 
and  is  said  to  be  absorbed.  No  substances  absorb  all  the  light. 

Dispersion. — Light  falling  upon  opaque  bodies  causes  them  to  emit 
light  in  all  directions,  and  thus  become  visible.  Such  bodies  are  said 
to  disperse  light,  because  they  scatter  it.  Light  is  thus  dispersed  by 
the  innumerable  little  facets  of  the  particles  composing  the  rough 
surfaces. 

Reflection. — Light  falling  upon  polished  surfaces  is  thrown  off  in  a 
regular  manner,  as  a  ball  rebounds  from  a  floor. 

Refraction  is  the  bending  of  a  ray  of  light,  caused  by  passing 
obliquely  from  one  transparent  medium  to  another. 


CATOPTRICS,    OR    REFLECTION    OF    LIGHT. 

Reflectors — Mirrors — Specula. 

365.  Reflectors  are  solid  bodies  bounded  by  regular  surfaces, 
highly  polished,  and  capable  of  reflecting  a  large  portion  of  the  light 
which  falls  upon  them. 

3 66.  Mirrors  and  specula. — Mirrors  are  reflectors  made  of 
glass  and  coated  with  an  amalgam  of  tin  and  quicksilver. 


OPTICS.  205 

Specula  are  reflectors  made  of  metal,  highly  polished.  Thirty-two 
parts  of  copper  to  fifteen  of  tin  make  the  best  metallic  reflectors. 

367 .  Forms  of  reflectors. — Reflectors  are  either  plane  or  curved. 
Curved  mirrors  or  specula  may  be  spherical,  elliptical,  or  paraboloid. 

A  concave  spherical  mirror  is  a  portion  of  the  interior  surface  of  a 
sphere.  A  convex  spherical  mirror  is  a  portion  of  the  exterior  surface 
of  a  sphere. 

A  line  passing  through  the  centre  of  and  perpendicular  to  a  spherical 
reflector,  is  called  the  axis  or  principal  axis.  The  centre  of  the  sphere, 
of  which  the  mirror  forms  a  part,  is  called  the  centre  or  optical  centre. 
The  middle  point  of  the  mirror  is  called  the  vertex.  For  paraboloid 
reflectors,  see  392. 

In  the  use  of  glass  mirrors,  a  portion  of  light  reflected  from  the  first 
surface  interferes  with  the  perfection  of  the  image  (398) ;  hence,  where 
the  most  perfect  instruments  are  required,  metallic  reflectors  are  em- 
ployed. 

368.  The  laws  of  reflection  of  light  are  the  same  as  those  of 
reflection  of  heat,  illustrated  by  Fig.  20,  Chart  4  (291),  and  are  thus 

stated : 
1st.  The  incident  ray,  the  perpendicular  at  the  point  of  incidence,  and 

the  reflected  ray,  are  all  situated  in  the  same  plane. 
2d.  The  angles  of  incidence  and  reflection  are  equal. 

369.  Direction  in  which  objects  are  seen.— Whenever  the 
light  passes  in  a  straight  line  from  the  object  to  the  eye,  the  object  will 
be  seen  exactly  where  it  is ;  but  when,  by  reflection  or  refraction,  the 
rays  are  turned  from  their  first  direction,  the  object  will  be  seen,  and  so 
appear  to  be,  in  the  direction  in  which  the  rays  are  -plG 
passing  at  the  point  where  they  enter  the  eye. 

Reflection  at  Plane  Surfaces. 

3^0.  Figure  3. — Reflection  of  diverg- 
ing rays. — In  accordance  with  the  second  law 
of  reflection  (368),  diverging  rays  before  reflection 
will  be  equally  divergent  after  reflection,  as  shown 
by  the  diagram,  in  which  A  is  the  plane  mirror. 

Reflection  from  a  plane  mirror,  therefore, 
changes  the  direction  of  the  rays,  and  removes 
the  point  of  apparent  convergence  or  divergence 
to  the  opposite  side  of  the  reflector.  The  dotted 
lines  show  the  course  which  the  rays  would  take 
if  the  mirror  were  removed. 


206 


OPTICS. 


371.  Figure  4. — Reflection  of  converging  rays. — In  ac- 
cordance with  the  same  law,  converging  rays  before  reflection  will  con- 
verge after  reflection.  The  degree  of  convergence  before  and  after 
reflection  will  be  the  same  ;  and  the  reflected  rays  will  meet  at  a  point 
as  far  in  front  of  the  mirror,  E,  as  the  point  at  which  they  would  meet 
(shown  by  the  dotted  lines)  is  behind  it,  if  the  mirror  were  removed. 


FIG.  4. 


FIG.  5. 


372.  Figure  5. — Reflection  of  parallel  rays. — In  accord- 
ance with  the  same  law,  parallel  rays  before  reflection  will  be  parallel 
after  reflection,  as  illustrated  by  the  diagram. 

373.  Figure  6.— Convex,  plane,  and  concave  mirrors.— 

A  ray  of  light  from  the  lamp,  T,  falling  upon  the  point,  H,  of  the 
plane  mirror,  KP,  will  be  reflected  to  the  eye  at  L.    The  line,  HN,4 

FIG.  6. 


being  perpendicular  to  all  three  of  the  mirrors  at  the  point,  H,  the 
same  ray,  TH,  in  accordance  with  the  second  law  of  reflection,  would 
be  reflected  to  the  same  point,  L,  by  each  of  the  three  mirrors. 


OPTICS. 


207 


374-  Fi&ure  7.— The  intensity  of  reflected  light  increases 
with  the  magnitude  of  the  angle  of  incidence.     If  the  light  of  the 
candle  be  reflected   by  a 
piece  of  paper  to  the  eyes, 
E  and  F,  it  will  be  more 
intense   at   E  than  at  F, 
because  the  angle  of  inci- 
dence  (THN,   Fig.  6)   is 
greater  at  B  than  at  A. 

This  is  illustrated  by  the 
fact,  that  near  sunset  the 
reflected  rays  of  the  sun 
are  so  brilliant,  the  eyes 
can  hardly  bear  to  look  at 
them,  while  at  mid-day 
we  observe  them  without  difficulty. 

Highly-polished  metallic  surfaces,  however,  reflect  more  light  as  the 
angle  of  incidence  diminishes,  the  greatest  amount  being  reflected 
when  the  incident  rays  are  perpendicular  to  the  surface. 

The  intensity  of  the  reflected  light  will  also  depend  upon  the  nature 
and  degree  of  polish  of  the  reflector. 

The  most  perfect  reflector  does  not  reflect  all  the  light,  but  diffuses 
a  part  of  it. 

FIG.  8. 


375.  Figure  8.— -Images  formed  by  plane  reflectors. — 
Virtual  image. — Let  AD  be  an  object  placed  in  front  of  a  plane 
horizontal  mirror.  The  light  from  the  points,  A  and  D,  will  be  re- 
flected to  the  eye  at  T.  The  rays  from  A  will  appear  to  come  from  a 


208 


OPTICS. 


point,  V,  as  far  behind  the  mirror  as  A  is  in  front  of  it.  The  same  is 
true  of  the  point,  D,  and  every  other  point  of  the  object.  Hence,  the 
object  will  appear  to  be  situated  as  far  behind  as  it  actually  is  in  front 
of  the  reflector. 

If  we  stand  with  our  right  arm  toward  the  mirror,  the  image  will 
appear  to  have  the  left  arm  toward  the  mirror ;  or  the  image  is  reversed 
laterally.  This  is  because  the  image  of  each  point  is  as  far  behind  the 
mirror  as  the  point  of  the  object  is  in  front.  The  image  will  also  be 
erect  and  full  size.  Hence,  the  object  and  image  are  symmetrically  sit- 
uated with  respect  to  the  mirror. 

The  virtual  image,  therefore,  is  an  image  which  appears  to  exist. 

If  a  person  approaches  a  mirror,  his  image  seems  to  come  forward  to 
meet  him. 


FIG.  9. 


376.  Figure  9. — Multiplicity  of  images  of  a  single  object 
seen  by  means  of  inclined  reflectors.     When  an  object  is  placed  between 

two  mirrors,  which  make 
with  each  other  an  angle  of 
90°,  or  less,  several  images 
are  produced,  varying  in  num- 
ber according  to  the  inclina- 
tion of  the  mirrors.  If  they 
are  perpendicular  to  each 
other,  three  images  will  be 
seen,  as  represented  by  the 
diagram.  Let  L  be  the  ob- 
ject, and  the  heavy  lines  the 
two  mirrors.  Place  the  di- 
viders at  N,  and  draw  the 
dotted  circle,  and  an  image 
of  the  object  will  be  seen  by 
the  eye  at  each  of  the  points, 

1,  2,  and  3.  The  plain  arrow-lines  represent  the  incident  and  reflected 
rays  by  which  the  object  is  seen.  The  straight  dotted  lines  show 
where  the  object  seem,s  to  be,  and  hence  the  three  images.  It  will  be 
noticed  that  the  image  3  is  the  result  of  double  reflection. 

377.  Kaleidoscope.— This  toy  depends  upon  the  multiplication 
of  images  by  inclined  mirrors,  as  just  explained;  the  mirrors  being 
placed  at  angles  varying  from  30°  to  60°,  and  inclosed  within  a  suit- 
able case  or  tube.     In  the  end  of  the  tube,  behind  ground  glass,  is  a 
narrow  cell,  containing  several  small  objects,  as  bits  of  colored  glass, 
tinsel,  etc.,  which  are  free  to  tumble  about. 


OPTICS. 


209 


FIG.  10. 


378.  Figure  10.— Deceptions  practised  by  use  of  mir- 
rors :  seeing  through  a  brick. — This  is  done  by  means  of  four 
mirrors,  and  live  short  tubes 

joined  together  at  right  an- 
gles, as  represented.  The 
mirror*,  1,  2,  3,  and  4,  are 
placed  successively  at  an  an- 
gle of  45°  to  the  incident 
ray  coming  from  the  object. 
The  rays  of  the  candle  are 
reflected  by  the  mirror  1, 
vertically  to  the  mirror  2; 
and  from  the  mirror  2,  hori- 
zontally to  3 ;  and  from  3, 
vertically  to  4 ;  and  from  4, 
horizontally  to  the  eye.  Thus  the  candle  will  be  seen,  apparently,  in  a 
direct  line  passing  through  the  brick,  or  other  opaque  body,  A. 

379.  Figure   11.— Plane    mirrors  may  reflect  objects 
double  their   own  length. — Let  D  be  the  mirror,  of  half  the 
length  of  the  object,  ET.     The  point  of  the  arrow  is  reflected  from  the 
top  of  the  mirror  back  to  the  eye,  the  reflected  ray  taking  the  track  of 
the  incident  ray.     The  opposite  extremity  of  the  object  is  reflected  by 

FIG.  11. 


the  lower  end  of  the  mirror,  the  incident  and  reflected  rays  forming 
equal  angles  with  the  surface  of  the  mirror.  In  the  same  manner  all 
points  of  the  object  will  be  reflected  to  the  eye ;  and  the  object  will 
appear  to  be  at  LF,  a  distance  as  far  behind  as  it  is  in  front  of  the 
mirror,  as  shown  by  the  dotted  lines,  which  are  imaginary  continua- 
tions of  the  incident  and  reflected  rays. 

14 


OPTICS. 

12. — The  mariner's  sextant.— This  is  an  in- 
measuring  the  altitudes  and  angular  distances  of  the 
v  uoaies,  and  depends  on  reflection  from  two  mirrors. 
The  mirrors,  A  and  L,  are  so  mounted  that  their  angle  of  inclina- 
tion can  be  varied.    The  mirror,  A,  is  attached  to  the  movable  arm 
AF,  by  which  its  inclination  is  changed  by  moving  the  arm  along  the 
graduated  arc.     The  mirror,  L,  is  firmly  attached  to  the  frame  of  the 
instrument,  a  little  of  the  silvering  of  its  outer  portion  being  removed, 
so  that  the  eye  can  have  a  direct  view  of  the  horizon  or  other  objects. 

FIG.  12. 


The  mirror,  A,  is  turned,  by  the  arm  AF,  until  the  rays  of  any  object, 
as  the  sun,  moon,  or  a  star,  are  twice  reflected,  and  so  directed  to  the 
eye ;  when  they  will  coincide  with  the  rays  of  the  horizon  or  other 
object,  H,  seen  by  direct  vision,  and  from  which  the  angular  distance 
of  the  star  is  to  be  measured.  The  angular  distance  of  the  object  is 
shown  by  the  position  of  the  index-arm  on  the  graduated  arc.  The 
telescope,  T,  is  to  facilitate  accurate  observation. 

Reflection  at  Curved  Surfaces. 

381.  Figure  13.— Convex  spherical  mirrors  illustrated 
by  plane  mirrors. — A  convex  mirror  may  be  considered  as  made 
up  of  an  indefinite  number  of  infinitely  small  plane  mirrors.  If  E  be 
two  of  these  small  plane  mirrors,  which,  taken  together,  represent  a 
portion  of  a  convex  mirror,  the  effects  of  convex  mirrors,  in  changing 


OPTICS. 


211 


the  direction  of  the  rays  of  light  which  FIG.  13. 

fall  upon  them,  will  be  easily  under- 
stood. 

Let  the  two  arrows,  F,  be  parallel 
rays;  the  one  on  the  right  falling  upon 
the  mirror  on  the  right,  while  the  other 
ray  falls  upon  the  mirror  on  the  left. 
Now,  as  the  angles  of  incidence  and 
reflection  are  equal  in  both  cases,  the 
rays  after  reflection  cannot  be  parallel, 
but  divergent.  The  degree  of  diverg- 
ence will  depend  upon  the  angle  formed 
by  the  two  mirrors,  or  upon  the  amount  of  curvature  of  the  mirror. 
Hence,  parallel  rays,  falling  on  a  convex  mirror,  are  reflected  divergent ; 
converging  rays  are  reflected  less  convergent  or  parallel ;  and  diverging 
rays  are  rendered  more  divergent. 

382.  Figure  14.— Convex  spherical  mirrors.— Let  A  be 

the  optical  centre  of  the  mirror.     The  ray  2  is  perpendicular  to  the 
surface  of  the  mirror,  and,  FlG  14 

if  continued,  would  pass 
through  the  centre,  A. 
/rhis  line  is  called  the 
principal  axis  of  the  mir- 
ror. The  two  rays,  1  and 
3,  being  parallel  to  this, 
the  three  rays  would  fall 
on  a  plane  mirror  in  a  per- 
pendicular direction,  and 
be  reflected  in  the  direc- 
tion of  their  incidence. 
But,  owing  to  the  obliqui- 
ty of  the  convex  surface, 
the  parallel  rays  1  and  3 
will  be  reflected  divergent 
(as  shown  by  the  last  diagram).  Draw  the  dotted  lines,  H  and  T,  per- 
pendicular to  the  reflecting  surface,  and  they  will,  if  continued,  pass 
through  the  centre  A.  The  ray  1  will  be  reflected  to  E,  in  such  a 
direction  that  the  angle  of  reflection,  between  E  and  H,  will  equal. the 
angle  of  incidence,  between  1  and  H ;  and  so  with  the  ray  3.  If  E  and 
F  be  continued,  as  shown  by  the  dotted  lines,  they  will  meet  behind 
the  mirror  at  V.  Hence,  since  the  image  is  always  seen  in  the  direc- 
tion of  the  reflected  ray,  an  object  placed  at  1  and  3  would  be  seen 
at  V,  or  respectively  in  the  directions  of  E  and  F. 


212  OPTICS. 

The  point,  V,  is  the  principal  focus  of  the  mirror,  and,  for  parallel  rays, 
is  situated  equally  distant  from  the  mirror  and  its  geometrical  centre, 
A ;  and,  being  behind  the  mirror,  is  called  the  principal  virtual  focus. 

Converging  rays  would  seem  to  meet  at  a  point  between  the  principal 
focus  and  the  mirror ;  and  diverging  rays,  at  a  point  between  the  prin- 
cipal focus  and  the  geometrical  centre,  A. 

3 83.  The  apparent  size  of  an  object,  seen  by  direct  vision, 
depends  upon  the  magnitude  of  the  angle  formed  at  the  eye  by  the  rays 
of  light  coming  from  the  extreme  borders  of.  the  object;  therefore,  any 
cause,  as  reflection  or  refraction,  which  changes  the  direction  of  the 
rays  by  which  an  object  is  seen,  so  as  to  enlarge  or  diminish  this  angle, 
at  the  point  where  it  meets  the  eye,  will  render  the  apparent  size  of  the 
object  larger  or  smaller  than  the  object  appears  by  direct  vision. 

384-  -Figure  15.— Formation  of  images  by  convex  re- 
flectors.— Images  formed  ly  convex  reflectors  are  always  erect  and 
virtual,  and  smaller  than  the  object. 

FIG.  15. 


Let  AB  be  the  object,  and  LT  will  be  the  image,  which,  as  repre- 
sented, is  erect,  virtual,  and  smaller  than  the  object.  The  converging 
rays  from  the  extremities  of  the  object  fall  upon  the  mirror,  and  are 
reflected,  less  convergent,  back  to  the  eye ;  and,  as  the  object  is  seen  by 
these  reflected  rays,  it  will  appear  to  be  smaller  than  it  would  if  seen 
by  the  same  rays  before  reflection  (383) ;  which  is  shown  by  continuing 
the  reflected  lines  behind  the  mirror,  until  they  meet  the  two  long 
broken  lines,  drawn  from  the  extremities  of  the  object  to  the  centre,  0, 
and  perpendicular  to  the  surface  of  the  mirror. 


OPTICS. 


213 


385.  Images   formed    by  convex   mirrors  are   larger 
the  nearer  the  object  approaches  the  mirror,  and  vice 
versa  (Fig.  15). — Let  the  object,  AB,  be  removed  toward  the  mirror 
to  the  position  of  the  dotted  arrow,  and  the  reflected  rays,  represented 
by  the  dotted  lines,  will  form  a  larger  angle  at  the  eye  than  when  the 
object  was  in  the  former  position,  which,  of  course,  renders  the  image 
correspondingly  larger,  as  shown  by  the  FIG 

dotted  prolongation  of   the  first  image, 
LT. 

386.  Figure   16.— Concave  re- 
flectors the  reverse  of  convex  re- 
flectors.— The  surface  of  the  concave 
mirror,  like  that  of  the  convex,  Fig.  13 
(381),  may  be  considered  as  made  up  of 
small  plane   mirrors.     Concave  mirrors 
render  parallel  rays  convergent,  instead 
of  divergent ;  and  convergent  rays  more 
convergent,  instead  of  less ;  and  divergent 
rays  less,  instead  of  more  divergent ;  as 

illustrated  by  the  parallel  rays,  N,  being  reflected  convergent,  by  means 
of  the  two  small  plane  mirrors,  M,  set  at  an  angle  with  each  other,  to 
represent  a  portion  of  a  concave  mirror. 

FIG.  17. 


387.  Figure  17.— Formation  of  images  by  concave  re- 
flectors.— In  this  diagram  the  object  is  placed  between  the  mirror 


214 


OPTICS. 


and  principal  focus.     The  centre  of  the  mirror  is  at  0,  and  the  prin- 
cipal focus  is  shown  by  a  dot  between  the  eye  and  eyebrow. 

Let  AD  be  the  object  ;  and  the  converging  rays  AT  and  DL  will  be 
reflected  back  to  the  eye  more  convergent  ;  thus  enlarging  the  angle 
under  which  the  virtual  image  is  seen  at  NH,  in  the  direction  of  the 
reflected  rays;  thus  showing  that,  when  the  object  is  between  the 
mirror  and  principal  focus,  the  image  is  erect,  virtual,  and  larger  than 
the  object. 

388.  Figure  18.  —  Foci  of  concave  mirrors  for  parallel 
and  convergent  rays.—  Parallel  rays  falling  near  the  axis  of  a 
FIG  !g  concave  mirror  converge,  after  re- 

flection, to  a  point  equidistant 
between  the  mirror  and  the  centre 
of  the  sphere,  of  which  the  mirror 
forms  a  part. 

In  the  diagram,  T  represents  the 
centre  and  F  the  principal  focus. 
The  central  line  passing  through 
the  centre  and  focus  is  the  axis. 
The  dotted  lines  from  the  centre 
to  the  mirror  are  perpendicular  to 
the  mirror. 

Rays  emanating  from  the  prin- 
cipal  focus,   F,   will   be   reflected 
parallel   to  the   axis   and  to  each 
other  ;  and,  conversely,  rays  paral- 
lel to  the  axis,  after  reflection,  will  meet  at  the  principal  focus,  F. 

Converging  rays  and  virtual  focus  (Fig.  18).  —  If  the  radiant 
point  passes  from  the  principal  focus,  F,  toward  the  mirror,  as  to/, 
the  reflected  rays,  N  and  H,  will  diverge,  as  though  emanating  from  a 
point  behind  the  mirror  called  the  virtual  focus  ;  and  converging 
rays,  as  H  and  N,  falling  upon  the  mirror,  will  be  reflected  to  some 
point  between  the  mirror  and  the  principal  focus,  as  at  /. 

The  lines  N  and  /form  equal  angles  with  the  dotted  or  perpendicular 
line  between  them,  being,  respectively,  the  angles  of  incidence  and 
reflection,  according  to  which  way  the  ray  proceeds.  In  the  same  way 
the  parallel  rays  and  their  reflected  rays,  to  F,  form  equal  angles  with 
the  dotted  perpendiculars. 

3  89.  Figure  19.  —  Foci  of  concave  mirrors  for  divergent 
rays.  —  If  the  rays  of  light  emanate  from  some  point  of  the  axis,  as  A, 


OPTICS.  215 

not  infinitely  distant  from  the  mirror,  they  will  be  brought  to  a  focus, 
after  reflection,  at  some  point  of  the  axis  between  the  principal  focus 
and  the  centre  of  the  mirror.  The  candle  on  the  right  is  at  the  prin- 
cipal focus,  the  one  in  the  middle,  at  the  centre  of  the  mirror. 

FIG.  19. 


The  point,  A,  and  the  point  where  its  reflected  rays  meet  the  axis, 
are  called  the  conjugate  foci.  Conjugate  foci,  therefore,  are  any  two 
points  so  related  that  a  pencil  of  light,  emanating  from  either  one,  is 
brought  to  a  focus  at  the  other.  The  one  from  which  the  light 
emanates,  as  A,  is  called  the  radiant. 

As  the  dotted  lines,  crossing  at  the  centre  of  curvature,  are  perpen- 
dicular to  the  mirror,  if  the  radiant,  A,  be  moved  toward  the  centre  of 
curvature,  its  conjugate  focus  or  reflected  rays  will  also  approach  the 
centre  ;  otherwise  the  angles  of  incidence  and  reflection  would  not  be 
equal ;  and  when  the  radiant  reaches  the  centre,  the  conjugate  foci 
will  meet,  and  the  incident  rays  will  be  perpendicular  to  the  mirror 
and  be  reflected  back  to  the  centre  from  whence  they  came. 

Some  of  the  properties  of  conjugate  foci  are  as  follows : 

1.  If  the  radiant  approaches  the  mirror,  the  focus  recedes  from  it. 

2.  If  the  radiant  is  beyond  the  centre,  the  focus  is  between  the  centre 
and  principal  focus. 

3.  If  the  radiant  is  at  the  centre,  the  focus  is  also  at  the  centre. 

4.  If  the  radiant  is  between  the  centre  and  principal  focus,  the  focus 
is  beyond  the  centre. 

5.  If  the  radiant  is  at  the  principal  focus,  the  focus  is  at  an  infinite 
distance  ;  that  is,  the  reflected  rays  are  parallel. 

6.  If  the  radiant  is  between  the  principal  focus  and  the  mirror,  as  at 
/,  Fig.  18  (388),  the  rays  are  reflected  so  as  to  diverge,  and  on  being 
produced  backward  meet  at  a  point  behind  the  mirror,  which  will  be 
the  focus,  and  which  is  called  the  virtual  focus. 


216  OPTICS. 

390.  Secondary  axes.— Oblique  pencils.— If  the  radiant,  A 
(Fig.  19),  is  not  situated  in  the  principal  axis,  but  at  any  point  near  it, 
a  line  drawn  from  the  radiant  through  the  centre  of  curvature  will 
constitute  a  secondary  axis,  and  the  focus  of  the  oblique  pencil  of  rays 
diverging  from  the  radiant  will  be  found  in  this  secondary  axis ;  and 
the  radiant  and  focus  will  possess  properties  entirely  analogous  to  those 
above  explained  (389). 

391.  Figure  20.— Spherical  aberration  of  reflectors.— 
Caustics. — It  would  appear,  from  what  has  been  said,  that  concave 

Y  G  on  an(i  convex  spherical  mirrors  re- 

flect rays  to  single  points  called 
foci,  but  this  is  not  strictly  the 
case,  only  for  rays  falling  near  the 
vertex  or  centre  of  the  mirror. 

Let  TL  be  the  axis;  F,  the 
principal  focus ;  and  the  several 
parallel  lines,  rays  of  light.  It 
will  be  seen  that  the  axial  ray  is 
reflected  back  upon  itself,  the 
angle  of  incidence  and  reflection 
being  nothing ;  while  the  ray  next 
above  is  reflected  back,  sensibly, 
to  the  principal  focus,  forming  a 
slight  angle  of  incidence  and  re- 
flection;  but,  as  the  distance  from  the  vertex  increases,  the  angles  of 
incidence  and  reflection  increase  in  size,  and  the  reflected  rays  sensibly 
depart  from  the  principal  focus ;  so  that  the  outer  ray,  H,  will  be  re- 
flected to  L.  This  scattering  of  the  reflected  rays  along  the  axis,  from 
the  principal  focus  toward  the  mirror,  is  called  spherical  aberration  by 
reflection  ;  and  the  brilliant  surface  formed  in  space  by  the  crossing  of 
the  reflected  rays,  two  by  two,  is  called  a  caustic. 

The  want  of  clearness  or  distinctness  in  the  image,  caused  by  this 
aberration,  makes  it  necessary  to  construct  concave  and  convex  mirrors 
in  the  form  of  paraboloid  surfaces  (392). 

392.  Figure  21. — Paraboloid  reflectors. — The  nature  of  a 
parabolic  curve  is  such  that  rays  parallel  to  its  axis  will  be  reflected  to 
a  single  point,  called  the  focus.     Let  F  be  the  focus,  and  the  lines  FN 
and  HN  will  form  equal  angles  with  the  mirror,  and  the  line,  NH,  will 
be  parallel  to  the  axis ;  and  this  is  true  of  each  of  the  other  rays  drawn 
from  the  focus.     And,  conversely,  all  rays  parallel  to  the  axis,  LF,  will 
be  reflected  exactly  to  the  focus,  F.   Or,  if  a  line  be  drawn  perpendicular 


OPTICS. 

to  any  point  of  the  reflect-  FIG.  21. 

or,  as  LN,  the  angle  of  in- 
cidence, HNL,  will  equal 
the  angle  of  reflection, 
FNL. 

If  the  lines  parallel  to 
the  axis  be  intersected  by 
a  line  perpendicular  to  the 
axis,  then  each  of  the  par- 
allel lines  plus  its  reflected 
line,  as  HN  plus  KF,  will 
equal  each  of  the  other 
parallel  lines  plus  its  re- 
flected line. 

Parabolic  reflectors  are  employed  in  reflecting  telescopes,  and  on 
railroad  locomotives,  for  illuminating  the  track  at  night,  etc. 

3 93.  Figure  22.—  Formation  of  images  by  concave  mir- 
rors when  the  object  is  beyond  the  centre  of  curvature. — 

Let  1  be  the  centre  of  curvature  and  F  the  principal  focus.     The  lines 
Al  and  HI,  drawn  from  the  extremities  of  the  object  through  the 

FIG.  22. 


centre  of  the  mirror,  are  the  secondary  axes  (390),  in  which  the  ex- 
tremities of  the  image,  T,  will  be  formed,  at  a  distance  from  the  mirror 
equal  to  the  conjugate  foci  (389)  for  the  extreme  points  of  the  object. 

This  image  is  real,  inverted,  smaller  than  the  object,  and  placed  'be- 
tween the  centre  of  curvature  and  the  principal  focus. 

The  image  wiil  be  very  bright,  as  all  the  light  incident  upon  the 
mirror  will  be  gathered  into  a  small  space. 


218 


OPTICS. 


As  the  object  approaches  the  mirror,  the  image  recedes  from  it  and 
approaches  the  centre,  1,  in  the  same  manner  as  do  the  conjugate  foci. 
Therefore,  when  the  object  is  at  1,  or  the  centre,  the  image  will  be  as 
large  as  the  object.  When  it  is  at  any  point  between  the  centre  and 
the  principal  focus,  F,  it  will  be  reflected,  enlarged,  and  more  distant 
from  the  mirror  than  itself  (387) ;  and  when  it  arrives  at  F,  the  image  be- 
comes infinite,  the  rays  being  reflected  parallel  (see  Conjugate  Foci,  389). 

DIOPTRICS,     OK    REFRACTION    OF    LIGHT. 

3 94-  Figure  23. — Definitions. — The  diagram  represents  a  dish 
filled  with  water.     Refraction  is  the  deviation  or  bending  which  a  ray 
FIG  23.  of  light  undergoes  in   passing 

obliquely  from  one  medium  into 
another,  as  FLT. 

The  ray,  FL,  before   refrac- 
tion, is  called  the  incident  ray. 
The   point  L,  at  which   the 
ray  is  deviated  or  bent,  is  called 
the  point  of  incidence. 

The  ray,  LT,  after  deviation, 
is  called  the  refracted  ray. 

The  angle  formed  by  the  in- 
cident ray,  FL,  and  the  normal, 
LN,  at  the  point  of  incidence, 
L,  is  the  angle  of  incidence  ;  and 
the  plane  FLN  of  the  angle,  the 
plane  of  incidence. 
The  angle  formed  by  the  refracted  ray,  LT,  and  the  normal,  at  the 
point  of  incidence,  L,  is  tjie  angle  of  refraction  ;  and  the  plane  of  this 
angle,  the  plane  of  refraction. 

As  a  portion  of  the  incident  ray,  FL,  is  reflected  by  the  surface  of  the 
water,  let  LE  be  the  reflected  ray  ;  then  the  angle  NLE  is  the  angle  of 
reflection. 

Describe  the  dotted  circle,  of  any  convenient  size,  from  the  point  of 
incidence,  L,  and  draw  the  line  H,  from  the  intersection  of  the  circle 
and  incident  ray,  so  it  will  fall  perpendicular  to  the  normal  NL ;  and 
also  the  line  T,  perpendicular  to  the  normal. 

The  line,  H,  is  the  sine  of  the  angle  of  incidence  ;  and  the  line  T,  the 
sine  of  the  angle  of  refraction  ;  and  H  divided  by  T  is  invariably  the 
same  for  any  given  medium,  whether  the  angle  of  incidence  is  increased 
or  diminished. 

The  quotient  obtained  by  dividing  H  by  T  is  called  the  index  of 
refraction. 


OPTICS.  219 

The  index  of  refraction  varies  with  different  media.  For  light  passing 
from  air  into  water,  it  is  about  £;  from  air  into  glass,  f ;  from  air  into 
diamond,  f.  These  fractions  inverted  will,  of  course,  express  the  index 
of  refraction  for  light  passing  out  of  water,  glass,  and  diamond,  into  air. 

395.  Laws  of  refraction. — l.  When  light  passes  from  a  rare  to 
a  denser  medium,  it  is  refracted  toward  the  perpendicular  or  normal ; 
and,  conversely,  when  it  passes  from  a  dense  to  a  rarer  medium,  it  is 
refracted  from  the  perpendicular  or  normal. 

2.  The  planes  of  incidence  and  refraction  coincide,  loth  being  normal 
to  the  surface  separating  the  media  at  the  point  of  incidence. 

3.  The  sine  of  the  angle  of  incidence  bears  a  constant  ratio,  in  the 
same  medium,  to  the  sine  of  the  angle  of  refraction. 

396.  The  cause  of  refraction  is  a  change  in  the  elasticity  of 
the  ether  in  passing  from  one  medium  into  the  other,  which  causes  a 
change  in  the  velocity  of  the  ray.     The  density  and  elasticity  of  ether 
in  water  are  different  from  what  they  are  in  the  atmosphere,  so  that 
light  travels  faster  in  the  latter  medium  than  in  the  former,  which 
causes  the  ray,  on  passing  from  air  into  water,  to  bend  toward  the  nor- 
mal at  the  point  of  incidence. 

397.  Figure  24. — Refraction  by  parallel  strata  of  differ- 
ent media. — If  a  ray  of  light  passes  through  one,  two,  or  several 
plates  of  dense  media,  all  FIG.  24 

the  refracting  surfaces  be- 
ing parallel  planes,  the 
emergent  ray  is  parallel 
to  the  incident  ray. 

Let  EVW  be  three  me- 
dia of  greater  density 
than  air,  and  the  second 
more  dense  than  the  first, 
and  the  third  more  dense 
than  the  second.  A  ray 
of  light  from  the  candle, 
incident  at  the  foot  of  the 
perpendicular,  N,  will  be 
refracted  at  the  upper  sur- 
face of  E ;  and  at  the  upper  surface  of  V  it  will  be  again  refracted, 
and  at  the  upper  surface  of  W  it  will  be  still  further  refracted ;  but  on 
emerging  into  the  air  at  E,  it  will  be  parallel  to  the  incident  ray,  and, 
to  the  eye,  the  candle  will  appear  to  be  situated  at  the  extremity  of 
the  dotted  line  below  the  object. 


220 


OPTICS. 


398.  Figure  25.— Refraction  and  internal  reflection.— 

Double  reflection  of  mirrors.— When  light  falls  obliquely  upon 

FIG  25  a  transparent  medium,  as  a 

plate  of  glass,  it  will  be  di- 
vided in  various  ways.  Let 
the  light  from  the  candle 
fall  upon  the  plate  of  glass, 
F,  at  the  foot  of  the  normal, 
N.  At  this  point  a  portion 
will  be  absorbed  and  another 
portion  dispersed  (364),  and 
still  another  portion  will  be 
reflected  in  the  direction  of  1, 
and  the  balance  will  be  re- 
fracted to  E,  on  the  opposite 
surface  of  the  glass.  At  this 
point  it  is  further  divided. 
A  part  emerges  into  the  air 
and  is  again  refracted,  parallel  to  the  incident  ray;  and  the  balance  is 
reflected  back  to  L,  which  is  again  divided,  a  portion  emerging  into 
the  air  and  passing  to  2,  parallel  to  the  first  reflected  ray,  1,  and  the 
balance  is  reflected  back  parallel  to  the  first  refracted  ray,  and  so  on ; 
until,  by  absorption  and  emergence,  the  light  is  lost. 

In  general,  only  the  rays  1,  BA,  and  2,  have  sufficient  intensity  to 
be  visible  to  the  naked  eye.  If  the  lower  face  of  the  glass  plate  were 
silvered,  that  is,  if  the  plate  were  a  mirror,  most  of  the  light  would  be 
reflected  by  the  silvered  side,  and  pass  in  the  direction  of  the  line  2, 
which  would  give  the  principal  image  of  the  object ;  and  a  faint  image 

of  the  object  would  be 
formed  by  the  first  reflec- 
tion, and  be  seen  in  the 
direction  of  the  line  1. 

399.  Figure  26.— 
Refraction  and  total 
reflection. — When  light 
passes  from  a  dense  to  a 
rarer  medium,  the  angle 
of  refraction  is  greater 
than  the  angle  of  inci- 
dence (395),  and  when  the 
angle  of  refraction  is  90°, 
the  angle  of  incidence  is 


FIG  26. 


OPTICS.  221 

much  less.  For  water,  it  is  48°  35',  for  ordinary  glass  it  is  41°  49'; 
consequently,  when  the  angle  of  incidence,  in  the  case  of  water,  exceeds 
48°  35',  refraction  cannot  occur,  and  the  light  will  be  totally  reflected. 

Let  the  diagram  represent  a  glass  globe  half- full  of  water.  A  ray  of 
light  passing  from  1  to  L,  being  normal  to  the  surface  of  the  globe, 
suffers  no  refraction  there,  but  coming  to  the  surface  of  the  water  at  L, 
it  is  refracted  away  from  the  normal,  NV,  to  the  eye  at  A  (395).  The 
angle  of  incidence,  1LV,  being  less  than  48°  35',  the  angle  of  reflection, 
NLA,  will  be  greater  than  48°  35',  but  less  than  90°vas  shown  by  the 
sine  H,  being  less  than  the  radius  LA.  But  a  ray  from  the  candle,  2, 
forming  a  greater  angle  of  incidence  than  48°  35',  would  not  emerge 
from  the  water,  otherwise  the  sine  of  its  angle  of  refraction  would  be 
greater  than  the  radius,  LA,  which  is  impossible;  therefore  the  ray 
from  2  will  be  wholly  reflected  by  the  surface  of  the  water,  and  pass  to 
the  eye  T,  forming  an  angle  of  reflection,  TLV,  equal  to  the  angle  of 
incidence,  2LV. 

Suppose  the  ray,  between  the  rays  1  and  2,  to  form  an  angle  of  inci- 
dence of  just  48°  35',  then  the  angle  of  refraction  would  be  90°,  and 
the  refracted  ray  would  fall  on  the  surface  of  the  water,  and  the  sine 
of  the  angle  of  refraction  would  equal  the  radius  of  the  globe. 

This  kind  of  reflection,  at  the  surface  which  separates  two  media,  is 
called  internal  reflection,  or  total  reflection. 

This  is  the  only  way  in  which  total  reflection  occurs,  however  smooth 
reflecting  surfaces  may  be  made. 

It  is  impossible  to  see  the  bottom  of  a  pond  of  water  when  looked  at 
obliquely,  because  the  rays  return,  by  total  reflection,  to  the  water, 

instead  of  emerging  into  the  air. 

FIG.  27. 

400.  Figure  27. 
—Effects  of  refrac- 
tion on  the  rising 
and  setting  of  the 
heavenly  bodies. 
— Suppose  the  dotted 
line,  in  the  diagram, 
to  represent  the  hori- 
zon; the  several  cir- 
cles, the  atmosphere; 
and  F,  the  sun  in  its 
actual  position.  The 
rays  of  the  sun  enter- 
ing the  atmosphere  in 
straight  lines,  become  more  and  more  refracted,  until  they  reach  the 


OPTICS. 


FIG.  28. 


eye  in  the  horizon.  Now,  as  the  apparent  position  of  an  object  lies  in 
the  direction  in  which  the  light  from  it  passes  at  the  point  where  it 
enters  the  eye,  the  sun  will  be  seen  on  the  dotted  line  of  the  horizon 
when  it  is  actually  at  F,  below  it.  Hence,  heavenly  bodies,  by  the 
amount  of  this  refraction,  rise  earlier  and  set  later  than  they  would 
were  there  no  atmosphere  surrounding  the  earth. 

The  crimson  appearance  of  the  horizon,  at  sunset  and  sunrise,  is 
owing  to  the  fact  that  the  red  rays  of  the  sun,  being  the  least  refrangi- 
ble, are  the  first  to  appear  in  the  morning  and  the  last  to  disappear  at 
night. 

Jf,01.  Figure  28. — Refraction  by  dense  media  spreads 
out  the  light. — When  a  ray  of  ordinary  daylight  or  sunlight  is  re- 
fracted by  a  dense  transparent 
medium,  the  refracted  light  is 
not  confined  to  a  single  line, 
but  it  is  spread  out  into  a  fan- 
like  form,  as  represented  in  the 
figure. 

Let  S  be  the  incident  ray, 
and,  after  refraction  by  the 
prism  L,  it  will  have  the  form 
shown  by  the  several  lines  be- 
tween V  and  K.  The  different 
parts  of  the  refracted  pencil 
show  different  colors ;  the  most 

refracted  part,  V,  being  violet,  and  the  least  refracted,  R,  being  red. 
The  index  of  refraction  for  any  one  color  is  uniform  for  any  given 
medium,  but  the  index  in  the  same  medium  varies  for  the  different 
colored  light. 

402.  Figure  29.- 
Mirage. — Mirage  is  an 
atmospheric  phenomenon, 
caused  by  refraction  and 
total  reflection;  and  de- 
pends upon  the  different 
strata  of  the  atmosphere 
being  unequally  heated, 
which  causes  rays  coming 
from  distant  objects  to  be- 
come curved  in  their  pas- 
sage to  the  eye ;  and  some- 
times a  layer  of  atmo- 


FIG.  29. 


OPTICS.  223 

sphere  next  the  earth  becomes  a  reflector,  causing  total  reflection  (399) 
of  the  oblique  rays  falling  upon  it  ;  which  will  cause  objects  to  appear 
inverted,  as  if  reflected  from  water.  In  this  way  portions  of  the  earth, 
especially  on  deserts,  appear  to  the  traveller  as  lakes  and  ponds.  To 
heighten  the  illusion,  trees  are  often  seen  reflected  from  these  apparent 
sheets  of  water. 

In  the  diagram,  the  ray  of  light  passing  from  the  top  of  the  tower 
toward  T,  is  gradually  curved  by  the  unequal  refractive  power  of  the 
different  layers  of  unequally  heated  atmosphere,  the  lower  stratum 
being  most  heated  ;  and  when  the  ray  reaches  the  point  T,  its  obliquity, 
or  angle  of  incidence  on  the  stratum  of  air,  is  such  that  total  reflection 
takes  place  ;  causing  the  ray  to  be  turned  in  its  course  toward  the  eye  ; 
and  the  ray  seeming  to  come  from  the  direction  in  which  it  is  passing 
at  the  point  where  it  enters  the  eye,  will  give  the  object  the  appear- 
ance of  being  inverted,  as  if  reflected  by  water.  In  this  case  both  the 
tower  and  its  image  are  seen. 


Figure  30.  —  Looming-  is  an  atmospheric  phenomenon 
caused  by  extraordinary  refraction,  by  which  objects,  on  the  shores  of 
lakes  and  seas,  appear  to  be  thrown  up  higher  than  their  real  positions. 
It  is  most  common  in  very  hot  and  very  cold  countries,  and  where  the 
sea  and  land  are  more  equally  divided.  It  is  due  to  different  strata  of 
the  atmosphere  being  unequally  heated.  The  lower  layers,  in  this  case, 

FIG.  30. 


being  the  coldest,  cause  the  rays,  coming  from  a  distant  object,  to  be- 
come curved  upward  instead  of  downward  in  the  passage.  When  some 
stratum  of  the  air  acts  as  a  reflector,  as  previously  explained  (402),  the 
object  will  not  only  be  elevated  but  inverted,  as  shown  in  the  diagram. 


224  OPTICS. 

By  looming,  ships  have  been  seen  at  so  great  a  distance  that  the  cur- 
'vature  of  the  earth  would  render  it  impossible  for  them  to  be  but  par- 
tially seen  by  direct  vision.  By  this  means  the  French  coast,  for 
several  leagues  in  length,  has  been  seen  at  Hastings,  in  England,  fifty 
miles  distant. 

404-  Figure  31.    The  depth  of  water  rendered  appar- 
ently less  by  refraction.— This  is  shown  by  a  dish  of  water  and 
FlG  31  a  piece  of  coin.     Let  L  be  the 

piece  of  coin,  placed  in  the 
bottom  of  the  empty  dish; 
then  let  the  observer  take 
such  a  position,  that  the  side 
of  the  vessel  will  cut  off  his 
view  of  the  coin.  By  filling 
the  dish  with  water  the  coin 
will  be  brought  to  his  view, 
and  have  the  appearance  of 
being  situated  at  T,  in  the 
direction  of  the  refracted 
rays. 

In  this  way  the  bottoms  of  rivers,  ponds,  etc.,  seem  to  be  nearer  the 
surface  of  the  water  than  they  really  are;  sometimes  causing  people  to 
venture  into  water  too  deep  for  their  safety. 

It  is  by  refraction  that  oars,  spiles,  etc.,  when  partly  plunged  in 
water,  seem  to  be  bent  at  the  surface  of  the  water. 


Prisms  and  Lenses. 

405.  Figure  32. — Prisms  and  Lenses. — These  are  usually 
made  of  glass,  and  are  of  various  forms. 

A.  prism  is  a  refracting  medium,  bounded  by  three  or  more  (usually 
three)  plane  faces,  variously  inclined  to  each  other.  In  the  diagram, 
1  represents  a  section,  or  end-view,  of  a  triangular  prism.  The  angle 
formed  by  the  two  adjacent  faces,  through  which  a  ray  of  light  passes, 
is  called  the  refracting  angle  of  the  prism. 

A  lens  is  a  refracting  medium,  usually  glass  or  crystal,  bounded  by 
curved  surfaces,  or  by  one  plane  and  one  curved  surface. 

Lenses  are  bounded  by  spherical  surfaces,  or  by  one  spherical  and 
one  plane  surface. 

When  the  surfaces  of  lenses  are  of  different  kinds,  they  are  named  in 
reference  to  the  side  on  which  the  light  first  falls. 


OPTICS. 

A  plane  glass,  section  3,  is  a  plate  of 
glass  having  two  plane  surfaces,  parallel 
to  each  other. 

If  the  several  sections,  2,  4,  5,  6,  7,  8,  9, 
were  revolved  around  the  straight  line 
passing  through  them,  they  would  seve- 
rally describe  the  solid  lenses  they  are 
intended  to  represent. 

A  sphere,  shown  in  section  2,  has  all 
parts  of  its  surface  equally  distant  from  a 
certain  point  within,  called  the  centre. 

A  double  convex  lens,  4,  is  bounded  by 
two  convex  surfaces. 

A  double  concave  lens.  5,  has  two  con- 
cave surfaces  opposite  to  each  other. 

A  plano-convex  lens,  6,  has  its  first  sur- 
face plane  and  the  other  convex. 

A  plano-concave  lens,  7,  has  its  first  sur- 
face plane  and  the  other  concave. 

A  meniscus,  8,  has  one  surface  convex 
and  the  other  concave,  the  concave  sur- 
face being  the  least  curved. 

A  concave-convex  lens,  9,  has  its  first 
surface  concave  and  the  other  convex,  the 
concave  surface  being  the  most  curved. 

An  achromatic  combination  consists  of 
two  or  more  lenses  of  diiferent  kinds  of 
glass,  so  constructed  as  to  neutralize  the 
effect  of  dispersion  (438).  This  combina- 
tion is  of  great  importance  in  the  con- 
struction of  optical  instruments. 

Lenses  either  converge  or  diverge  rays 
of  light. 

Convergent  lenses  have  greater 
convexity  than  concavity,  and,  therefore, 
are  thicker  in  the  middle  than  at  their 
edges.  These  are  2,  4,  6,  and  8. 

Divergent  lenses  have  greater  con- 
cavity than  convexity,  and,  therefore,  are 
thinner  in  the  middle  than  at  their  edges. 
These  are  5,  7,  and  9. 

15 


225 


FIG.  32. 


226  OPTICS. 

406.  Figure  33. — Refraction  by  prisms. — Finding  the  di- 
rection of  the  refracted  and  emergent  rays. — Let  the  triangle  represent 
a  section  of  a  crown-glass  prism  whose  index  of  refraction  (394)  is  1.5 ; 
and  AL,  a  ray  of  light,  from  the  candle,  falling  obliquely  upon  the 
prism  at  L. 

FIG.  33. 


To  find  the  point  at  which  the  ray  will  emerge  from  the  prism  on  its 
opposite  face,  first  draw  the  dotted  line,  BF,  through  the  point  of  in- 
cidence, perpendicular  to  the  face  of  the  prism,  and  ALB  will  be  the 
angle  of  incidence,  which  will  be  to  the  angle  of  refraction  as  1.5  is  to 
1.  Now  erect  a  line,  which  (by  some  scale,  say  inches)  is  1.5,  perpen- 
dicular to  AL  at  a  point  where  it  will  meet  BL ;  then  describe  a  circle 
from  L,  passing  through  the  point  where  these  lines  meet.  On  LF 
erect  a  perpendicular  line  equal  to  1,  at  a  point  where  it  will  meet  the 
circle,  and  through  this  point  the  refracted  ray,  LT,  must  pass ;  for  the 
perpendicular  to  AL  is  the  sine  of  the  angle  of  incidence  and  the  per- 
pendicular to  LF  is  the  sine  of  the  angle  of  refraction,  and  these  are  to 
each  other  as  1.5  is  to  1. 

If  it  were  not  for  this  refraction,  which  has  turned  the  ray  toward 
the  perpendicular  BF,  it  would  have  passed  on  in  the  direction  of  TH. 

By  a  like  process,  TS  will  be  found  to  be  the  direction  of  the  ray 
after  emergence.  In  this  case,  as  the  ray  passes  from  a  dense  to  a  rarer 
medium,  it  will  be  turned  from  the  perpendicular  TN  (394)  toward  the 
face  of  the  prism. 

As  the  eye  views  the  object  by  the  emergent  ray  it  will  seem  to  be  at 
E,  in  the  direction  of  ST. 

By  slowly  turning  the  prism  backward  and  forward  about  its  axis, 
one  position  will  be  found  where  there  is  the  least  distance  between  the 
real  position  of  the  object,  as  at  A,  and  its  apparent  position,  as  at  E. 
This  position  of  the  prism  is  when  the  emergent  ray,  TS,  deviates  the 


OPTICS. 


227 


least  possible  from  the  incident  ray  AL,  which  will  be  the  case  when 
the  refracted  ray,  LT,  is  parallel  to  the  base  of  the  prism.  In  this 
position  the  angles,  at  which  the  ray  enters  and  leaves  the  prism,  will 
be  equal. 

Effects  of  a  plane-glass. — For  the  effects  of  a  plane-glass  upon 
an  oblique  ray  of  light,  see  Figs.  24=  and  25  (397-8). 

Jf07-  Figure  34.— The  course  of  light  through  a  sphere 
of  glass  or  spherical  lens. — Let  AB  represent  three  parallel  rays 
of  light  incident  upon  the  crown-glass  spherical  lens,  represented  by 
the  circle,  the  middle  ray  passing  through  its  centre,  L ;  and  let  HTL 
be  a  perpendicular  to  the  surface  at  T.  The  ray  AT,  on  entering  the 

FIG.  34. 


lens,  will  be  refracted  toward  the  perpendicular.  From  the  point  T 
describe  the  arcs,  as  shown,  one  of  which  passes  through  the  centre  of 
the  circle;  then  draw  TF,  in  such  a  direction  that  the  sine  of  the  angle 
of  incidence,  ATH,  will  be  to  the  sine  of  the  angle  of  refraction,  FTL, 
as  1.5  is  to  1 ;  or,  as  the  index  of  refraction  of  crown-glass  is  to  that  of 
air. 

By  the  same  process,  and  the  perpendicular,  LF,  the  direction  of  the 
emergent  ray  may  be  found ;  which,  coming  from  a  dense  to  a  rarer 
medium,  will  be  turned  from  the  perpendicular  LF,  and  pass  to  the 
eye,  where  it  will  meet  the  ray,  B,  and  with  it  form  the  angle  under 
which  the  eye  would  view  an  object,  whose  real  position  and  magnitude 
is  AB,  but  whose  apparent  magnitude,  of  course,  would  be  much  larger. 
The  central  ray,  being  normal  to  the  sphere,  passes  through  it  without 
deviation. 

The  point  where  A  and  B  meet  is  the  focus  for  parallel  rays. 

To  find  the  distance  of  the  focus  from  the  centre  of  the  lens,  divide 


22$  OPTICS. 


the  index  of  refraction,  of  the  material  of  which  the  lens  is  made,  by 
twice  its  excess  above  1  —  the  radius  of  the  lens  being  1. 


Action  of  Convex  Lenses. 


408.  Figure  35.—  Definitions.—  The  centres  of  the  bounding 
surfaces  of  a  lens  are  called  centres  of  curvature  ;  thus,  the  centres  of 


FIG.  35. 


curvature  of  the  several  lenses  in  the  diagram,  are  the  centres  of  the 
two  circles. 

The  axis  of  the  lens  is  the  straight  line  drawn  through  the  centres 
of  curvature,  as  LM,  for  the  lens  H. 

In  higher  optics,  it  is  demonstrated  that  there  is  always  one  point  on 
the  axis  of  a  lens,  such,  that  rays  of  light,  passing  through  it,  are  not 
deviated  by  the  lens.  This  point  is  called  the  optical  centre,  and  is  of 
much  use  in  the  construction  of  images. 

If  the  surfaces  of  double  convex  and  double  concave  lenses  are 
equally  curved,  the  optical  centre  is  on  the  axis,  midway  between  the 
two  surfaces,  as  H ;  and  any  ray,  as  EF,  passing  through  it,  is  not 
deviated  by  the  lens. 

To  find  a  normal  at  any  point  of  the  surface  of  a  lens,  draw  a  line 
through  that  point  to  the  corresponding  centre  of  curvature  ;  thus,  the 
dotted  lines,  S  and  N,  are  normals  to  the  points  where  they  enter  the 
lens. 

Action  of  convex  lenses  on  light  (Fig.  35). — A  ray  of 
light  falling  upon  one  surface  of  a  double  convex  lens  is  refracted  to- 


OPTICS.  229 

the  normal ;  and,  passing  through  the  lens,  is  again  incident 
upon  the  other  surface,  and  is,  therefore,  refracted  from  the  normal ; 
the  deviation,  in  both  cases,  being  toward  the  thicker  portion  of  the 
lens,  which  is  analogous  to  the  action  of  the  prism  ;  see  Fig.  38  (411). 
Therefore,  rays  of  light,  as  W  and  A,  parallel  to  the  axis,  will  be  col- 
lected, by  refraction,  to  a  single  point,  called  the  principal  focus  ;  and 
its  distance  from  the  lens  is  called  the  principal  focal  distance. 

If  the  lens  be  made  of  glass,  whose  index  of  refraction  is  1.5,  then 
the  principal  focus,  for  double  convex  lenses,  as  T  (Fig.  35),  is  the 
centre  of  curvature.  Hence,  the  principal  focal  distance  is  equal  to 
the  radius  of  the  curvature. 

The  principal  focus  for  plano-convex  lenses,  as  N,  is  on  the  axis  at  a 
point  in  the  circle  of  which  the  convex  surface  of  the  lens  forms  a  part. 
Hence,  the  principal  focal  distance  is  equal  to  twice  the  radius  of  the 
curvature,  as  represented  in  the  diagram. 

409.  Figure  36. — Conjugate  Foci. — These  are  any  two  points 
on  the  axis  of  a  lens,  so  situated  that  a  pencil  of  light  from  one  is 
brought  to  a  focus  at  the  other.  The  radiant  is  the  one  from  which  the 
light  proceeds. 

Let  the  two  white  dots  be  the  principal  foci  of  the  lens,  that  is,  for 

FIG.  36 


parallel  rays.  If  the  radiant,  L,  be  situated  beyond  the  principal 
focus,  as  it  is,  the  diverging  rays  will  be  brought  to  a  point  at  the  eye, 
somewhere  beyond  the  principal  focus  on  the  right.  These  two  points, 
L  and  the  eye,  are  conjugate  foci ;  and  it  matters  not  upon  which  side 
of  the  lens  the  radiant  is  placed. 

If  the  radiant  be  placed  at  twice  the  distance  of  the  principal  focus 
from  the  lens,  the  corresponding  focus  will  be  equally  distant  from  the 
lens ;  or,  the  points  at  which  the  conjugate  foci  will  be  equally  distant 
from  the  lens  is  when  either  of  them  is  double  the  distance  of  the 
principal  focus. 

If  the  radiant  is  at  an  infinite  distance  the  rays  are  parallel ;  in 
which  case  the  corresponding  focus  will  coincide  with  the  principal 
focus. 


230 


OPTICS. 


410.    Figure    37. — Conjugate    foci,   continued. — As   the 

radiant  approaches  the  principal  focus  on  its  side  of  the   lens,  the 
corresponding  focus  will  recede  from  the  principal  focus  on  the  opposite 


FIG.  37. 


side,  as  shown ;  and  when  the  radiant,  L,  reaches,  or  coincides  with  the 
principal  focus,  the  corresponding  focus,  (the  eye)  will  be  at  an  infinite 
distance,  as,  in  this  case,  the  refracted  rays  will  be  parallel. 

If  the  radiant  is  nearer  to  the  lens  than  the  principal  focus,  the  rays 
will  diverge,  as  shown  by  the  lines  A  and  B  (Fig.  38),  and  -will  meet 
only  by  being  produced  backward,  as  at  E ;  in  which  case  the  focus  is 
virtual,  the  radiant  being  at  the  intersection  of  the  lines  A  and  B. 

If  the  radiant  be  placed,  instead  of  on  the  axis,  on  any  line  (not 
much  inclined  to  the  axis)  passing  through  the  optical  centre  (408), 
called  a  secondary  axis,  the  corresponding  focus  will  be  on  that  line, 
and  the  laws  which  regulate  the  positions  of  conjugate  foci,  already  ex- 
plained, will  be  applicable. 

411.  Figure  38.— Analogous  effects  of  prisms  and 
double  convex  lenses,  shown  by  their  action  on  diverging,  paral- 
lel, and  converging  rays. 


Within   the  double    convex  lens  draw  sections  of    two   prisms    so 
that  their  angles  of  refraction  shall  coincide  with  the  edges  of  the 


OPTICS.  231 

lens,  and  their  opposite  sides  with  the  axis  of  the  lens,  as  repre- 
sented. 

Let  the  rays,  H  and  K,  be  parallel  to  the  axis,  and,  by  the  lens,  they 
will  be  refracted  to  its  principal  focus,  F  ;  the  diverging  rays,  L  and 
N  (coming  from  beyond  the  principal  focus  on  the  left),  will  be  re- 
fracted to  some  point  beyond  the  principal  focus  on  the  right;  and  the 
converging  rays,  A  and  B,  will  be  refracted  to  some  point  between  the 
principal  focus,  F,  and  the  lens. 

If,  now,  the  lens  be  removed,  and  the  inscribed  prisms  substituted, 
they  will  have  the  same  effect  as  the  lens  upon  the  several  rays  of  light. 


Figure  39.  —  Longitudinal  spherical  aberration  of 
lenses,  and  the  principles  determining  the  foci  of  lenses.  A  double 
convex  lens  may  be  regarded  as  composed  of  a  number  of  segments  of 


FIG.  39. 


prisms,  as  illustrated  by  the  lower  half  of  the  lens  in  the  diagram.  The 
further  the  segment  is  from  the  centre  of  the  lens,  the  greater  is  the 
inclination  of  its  faces,  and,  therefore,  the  greater  will  be  the  refraction 
of  the  rays  passing  through  it.  The  central  segments  may  be  regarded 
as  a  plane  glass,  with  nearly  parallel  faces,  while  the  outer  segments  ap- 
proximate the  form  of  prisms. 

Now,  since  the  deviation  of  a  ray,  passing  through  a  prism,  increases 
as  the  inclination  of  the  faces  of  the  prism  increases,  the  rays  L  and 
N  will  be  refracted  more  than  the  two  rays  passing  through  the  central 
segments ;  therefore  the  rays,  as  L  and  N,  passing  through  the  edges 
of  the  lens,  will  meet  nearer  to  the  lens,  as  at  F,  than  those  passing 
through  the  central  portion  of  the  lens,  which  will  meet  at  a  greater 
distance  from  the  lens,  as  at  E  ;  while  all  intermediate  rays  will  meet 
at  various  points  along  the  axis  between  F  and  E. 

This  scattering  of  focal  points  of  different  rays  along  the  axis,  from 
F  to  E,  is  called  longitudinal  spherical  aberration,  and  is  provided 


232  OPTICS. 

against  by  giving  the  faces  of  lenses  a  peculiar  curve;  but  owing  to  the 
difficulty  of  grinding  them  in  this  form,  the  aberration  is  practically 
obviated  by  making  the  lenses  relatively  thin,  which  diminishes  the 
inclination  of  their  faces. 

A  plano-convex  lens  has,  in  general,  the  same  effect  as  the  dou- 
ble convex  lens,  only  its  foci  are  at  double  the  distance ;  the  principal 
focus  being  at  a  distance  equal  to  twice  the  radius  of  the  curved  sur- 
face (408). 

Formation  of  Images  by  Convex  Lenses. 

« 

413.  Figure  40.— Formation  of  images  by  convex  lens- 
es, when  the  object  is  twice  the  focal  distance. — If  an  object, 

as  AE,  be  placed  before  a  lens,  all  points  of  it,  on  either  side  of  the 
principal  axis,  may  be  regarded  as  radiants,  situated  on  secondary  axes, 
sending  out  pencils  of  rays.  For  instance,  let  A  and  E  be  two  such 

FIG.  40. 


radiant  points.  Draw  the  secondary  axes  AB  and  EF,  and  let  the  two 
dots  on  the  principal  axis  be  the  principal  foci  of  the  lens.  The  point 
A  being  beyond  the  principal  focus,  its  rays  will  meet  on  its  secondary 
axis,  at  B,  beyond  the  principal  focus  on  the  right;  and,  in  the  same 
way,  the  rays  from  E  will  meet  on  its  secondary  axis  at  F;  and  so  on, 
for  all  points  of  the  radiant  EA. 

As  all  the  secondary  axes  cross  the  principal  axis,  the  image,  BF,  will 
be  inverted. 

The  size  and  position  of  the  image  will  depend  upon  the  distance  of 
the  object  from  the  lens. 

In  this  case,  the  object,  EA,  is  at  twice  the  distance  of  the  principal 
focus,  and,  consequently,  according  to  the  law  regulating  the  positions 
of  conjugate  foci  (409  and  410),  the  image,  BF,  must  be  at  an  equal 
distance  from,  and  on  the  opposite  side  of,  the  lens,  real  and  virtual. 

4 14-  Figure  41  .—Images  formed  by  convex  lenses  when 
the  object  is  at  more  and  less  than  twice  the  focal  dis- 
tance.— 1.  Let  the  two  dots  on  the  principal  axis  be  the  principal 


OPTICS. 


233 


foci;  and  AB,  the  object,  situated  more  than  twice  the  focal  distance 
from  the  lens,  in  which  case  the  image,  HN,  will  be  smaller  than  the 
object,  real,  inverted,  and  situated  betiveen  the  principal  focus  and  the 
point  at  twice  the  focal  distance. 


FIG.  41. 


2.  If  HN  be  the  object,  situated  between  the  principal  focus  and 
twice  the  focal  distance,  then  the  image,  AB,  will  be  larger  than  the 
object,  real,  inverted,  and  situated  at  more  than  twice  the  focal  distance. 

If  AB,  as  an  object,  be  moved  toward  the  lens,  the  image,  HN,  will 
grow  larger  and  recede  from  the  lens.  If  AB  be  moved  from  the  lens, 
the  image,  HN,  will  move  toward  the  leris  and  become  smaller. 

The  image,  however,  can  never  approach  nearer  to  the  lens  than  the 
principal  focus,  as  this  is  the  focus  for  parallel  rays,  in  which  case  the 
object  would  be  at  an  infinite  distance. 

The  linear  magnitude  of  the  image,  as  compared  with  the  object,  will 
be  proportional  to  their  respective  distances  from  the  lens. 


.  Figure  42.  —  Images  formed  by  convex  lenses  when 
the  object  is  at  less  than  the  focal  distance.  —  If  the  object, 


FIG.  42. 


NH,  and  the  eye,  be  placed  nearer  to  the  lens  than  the  principal  foci, 
FF,  the  image,  AB,  will  increase*  will  be  erect  and  virtual. 


234  OPTICS. 

The  image  being  virtual  can  only  be  seen  by  looking  through  the 
lens  ;  while,  in  the  former  cases,  the  images,  being  real,  would  be  seen 
on  an  intercepted  screen. 

In  this  case,  the  lens  becomes  what  is  called  a  single  microscope. 

The  rays  from  the  object  are  refracted  to  the  eye  by  the  lens,  and 
the  eye  will  see  the  image,  AB,  in  the  direction  of  the  dotted  lines. 


Figure  43.  —  Light-houses.  —  These  are  towers  erected 
along  the  coast,  upon  the  tops  of  which  are  large  lanterns,  lighted, 
at  night,  as  guides  to  mariners. 

In  early  times  light-houses  were  illuminated  by  fires,  made  of  wood, 
coal,  or  other  substances;  subsequently  by  means  of  oil-lamps,  placed 
in  foci  of  concave  reflectors.  But  the  metal  reflectors,  becoming  tar- 
nished by  sea-air,  soon  lost  much  of  their  reflective  power,  which  led 
to  the  invention  of  a  new  system  of  illumination,  which  is  being 
adopted  in  all  civilized  countries. 

FIG.  43. 


This  consists  of  substituting,  for  the  reflectors,  plano-convex  lenses, 
in  the  principal  foci  of  which  are  placed  powerful  lamps,  with  several 
concentric  wicks.  The  difficulty  of  making  large  plano-convex  lenses, 
together  with  their  great  absorption  of  light,  led  to  the  adoption  of  a 
system  of  lenses,  known  as  echelon  lenses. 

A  lens  of  this  kind  is  represented  by  the  diagram ;  A  (on  the  left) 
being  a  front  view,  and  (in  the  main  figure)  a  side  view.  It  consists 
of  a  plano-convex  lens  in  the  centre,  a  foot  or  so  in  diameter,  around 


OPTICS.  235 

which  is  disposed  a  series  of  plano-convex  annular  lenses,  with  such  a 
curvature  that  each  shall  have  the  same  principal  focus  as  the  central 
lens,  A.  Around  about  this  compound  lens  are  several  ranges  of  small 
reflectors,  M,  so  arranged  as  to  reflect  such  light  as  would  otherwise 
be  lost. 

This  double  combination  sends  forth,  in  a  single  direction,  an  im- 
mense beam  of  light,  as  shown  in  the  diagram,  which  is  visible  for  fifty 
or  sixty  miles. 

To  make  the  light  visible  in  more  than  one  direction,  eight  such  sys- 
tems are  arranged  on  different  sides  of  the  lamp ;  which,  for  light- 
houses of  the  first  order,  present  an  appearance  of  a  pyramid  of  glass, 
nine  or  ten  feet  high. 

To  make  the  light  visible  in  all  points  of  the  horizon,  the  whole  is  set 
upon  a  vertical  shaft  or  spindle ;  which,  by  the  employment  of  machi- 
nery, like  clock-work,  is  made  to  revolve.  By  this  means  an  observer 
at  any  point  will  see  eight  flashes  of  light  during  one  revolution,  which 
are  followed  by  as  many  intervals  of  darkness,  called  eclipses. 

One  light-house  is  distinguished  from  another  by  varying  the 
revolutions  of  the  lights. 

Concave  Lenses. 

417.  Figure  44. — Effects  of  concave  lenses  on  rays  of 
light. — Lenses  of  greater  concavity  than  convexity  (405)  render  paral- 
lel rays  divergent ;  converging  rays,  less  convergent ;  and  diverging 

FIG.  44 


rays,  more  divergent.  The  divergent  rays  from  the  radiant  L,  by  pass- 
ing through  the  double  concave  lens,  are  rendered  more  divergent,  and 
take  the  directions  of  M  and  N,  as  though  proceeding  from  a  point 
behind  the  lens,  as  shown  by  the  dotted  lines.  This  point  is  called  the 
virtual  focus. 


236 


OPTICS. 


The  parallel  rays,  S  and  P,  would  become  divergent,  and  the  converg- 
ing rays,  Y  and  W,  would  become  less  convergent,  on  passing  through 
the  lens.  The  principal  focus,  F,  is  the  centre  of  curvature. 

418.  Figure   45. — Formation   of    images    by   concave 

lenses. — Let  F  be  the  centre  of  curvature,  and  AB  the  object.    A 

pencil  of  light  coming  from  A,  is  deviated,  and  appears  to  come  from 

,  the  top  of  the  image,  H,  situated  on  the  secondary  axis  or  line  drawn 

FIG.  45. 


from  A  to  the  optical  centre  of  the  lens.  A  pencil  of  rays  coming  from 
B,  is  deviated,  so  as  to  appear  to  come  from  the  bottom  of  the  object, 
H,  situated  on  the  secondary  axis  or  line  drawn  from  B  to  the  optical 
centre  of  the  lens.  Therefore,  H  is  the  image  of  the  object,  AB.  Hence, 
images  formed  by  concave  lenses  are  erect,  virtual,  and,  being  nearer  the 
optical  centre,  smaller  than  the  object. 


CHROMATICS,    AND    DECOMPOSITION    OF    LIGHT. 

The  Solar  Spectrum. 

Figure  46.— Solar  spectrum.— Primary  colors.— A 

beam  of  sunlight  let  into  a  dark  room,  through  a  small  hole  in  the 
shutter,  and  passing  through  a  triangular  prism,  P,  will  be  twice  re- 
fracted out  of  its  course,  instead  of  passing  on,  as  to  T ;  and  instead 
of  being  refracted  to  a  single  round  point,  on  an  intercepting  screen, 
it  will  be  diffused  or  spread  out  over  a  considerable  space,  from  V  to  K, 
called  the  solar  spectrum,  in  which  will  be  seen  all  the  colors  of  the 
rainbow.  This  dispersion  is  owing  to  the  unequal  refrangibility  of 
the  different  colors.  Beginning  with  the  color  least  refracted,  they 


OPTICS. 


237 


are  red,  orange,  yellow,  green,  blue,  indigo,  and  violet,  as  shown  in  the 
drawing. 

FIG.  46. 


The  property  which  a  refractive  medium  possesses  of  decomposing 
and  scattering  solar  light,  is  called  its  dispersive  power.  The  dispersive 
power  of  different  substances  varies.  For  example,  the  spectrum  formed 
by  flint  glass  is  nearly  twice  as  long  as  that  formed  by  crown  glass. 

Primary  colors.  —  If  a  hole  be  cut  through  the  screen  opposite 
any  one  of  these  colors,  and  the  light  allowed  to  pass  through  and  fall 
upon  another  prism,  it  is  found  that  it  can  be  further  refracted,  but  it 
cannot  be  further  decomposed  or  separated  into  other  colors  by  refrac- 
tion. Hence,  the  colors  of  the  solar  spectrum  are  generally  called  pri- 
mary colors. 


.  Properties  of  the  solar  spectrum  (Fig.  46).—  Whether 
or  not  light  and  heat  are  one  and  the  same,  or  whether  their  difference 
consists  only  in  the  rate  or  velocity  of  the  vibrations  of  the  ethereal 
medium,  yet  it  is  evident  that  there  are  three  classes  of  effects  produced 
by  the  solar  radiations,  which  are  widely  different,  namely:  luminous 
effects,  which  act  upon  the  eye  ;  thermal  or  calorific  effects,  which  ex- 


238  OPTICS. 

pand  all  bodies ;  and  chemical  effects,  which  play  between  different 
elementary  substances,  causing  chemical  changes. 

These  three  kinds  of  force,  being  unequally  refrangible,  are  separated, 
and  so  examined,  by  means  of  the  refractive  power  of  the  prism,  in  the 
manner  previously  shown  for  separating  the  colors  of  light  (419). 

By  referring  to  the  diagram  (Fig.  46)  it  will  be  noticed  that  from 
the  prism,  P,  to  the  spectrum,  SS,  there  are  three  kinds  of  lines, 
which  represent  these  three  forces,  agents,  or  properties  of  the  solar 
rays.  Of  course,  they  are  not  distinctly  separated  from  each  other,  as 
the  lines  are,  but  they  blend  together  like  the  colors  in  the  spectrum 
or  rainbow. 

The, plain  lines  represent  the  luminous  rays;  the  dotted  lines,  the 
chemical  rays ;  and  the  broken  lines,  the  calorific  rays.  The  beam  of 
sunlight,  therefore,  instead  of  being  separated  into  only  seven  rays,  is 
decomposed  into  twenty-one  rays ;  seven  luminous,  lighting  or  color- 
ing ;  seven  calorific  or  heating ;  and  seven  chemical  rays. 

These  three  forces  or  agents  are  not  equally  powerful  in  all  parts  of 
the  spectrum.  There  are  points  where  each  has  a  maximum  and  mini- 
mum intensity. 

Below  the  red  ray,  quite  out  of  the  color,  is  the  most  powerful 
calorific  ray  (represented  by  the  first  broken  line  below  R),  each  calor- 
ific ray  diminishing  in  intensity  of  heat  as  we  pass  up,  and  lying  just 
below  its  respective  lighting  ray.  On  the  contrary,  the  most  powerful 
chemical  ray  is  at  the  top,  above  the  violet,  and  quite  out  of  the  color, 
each  chemical  ray  diminishing  in  intensity  of  effect  as  we  pass  down, 
until  we  reach  the  strongest  lighting  ray  (which,  as  will  be  seen,  is  the 
yellow),  where  the  chemical  effect  diminishes  to  nothing  ;  then  increas- 
ing again,  and,  finally,  diminishing  to  nothing  at  the  lower  extremity 
of  the  spectrum ;  thus  showing  two  maxima  of  chemical  influence. 

The  most  powerful  coloring  or  lighting  ray  is  the  yellow,  the  lighting 
effect  diminishing  from  this  color  to  nothing  at  either  end  of  the  spec- 
trum. 

The  chemical  effect  extends  as  high  as  the  dotted  line  at  the  extreme 
upper  end  of  the  spectrum,  S,  and  the  heating  effect  as  low  as  the 
broken  line  at  the  extreme  point  of  the  spectrum,  S,  below. 

By  experiment  it  has  been  found  that  the  change  wrought  within 
the  vegetable  leaf,  namely,  the  de-oxidization  of  carbon  and  hydro- 
gen, or  the  decomposition  of  carbonic  acid  and  water,  by  which  oxygen 
is  liberated,  takes  place  with  far  the  greatest  activity  in  the  yellow  ray, 
where  the  light  is  most  intense  and  the  chemical  effect  is  least.  Now, 
notwithstanding  the  point  of  greatest  intensity  of  this  de-oxidizing 
force,  agent,  or  property  of  the  solar  rays  corresponds  with  that  of  the 
lighting  or  coloring  rays ;  yet  its  effect  on  vegetation  is  so  different 


OPTICS.  239 

from  that  of  light  upon  the  eye,  it  is  not  improbable  that  this  is  a  dis- 
tinct property  of  solar  radiation. 

The  intensity  of  heat,  in  different  parts  of  the  spectrum,  may  be 
tested  by  a  delicate  thermometer  ;  the  intensity  of  chemical  effect  by 
paper,  previously  prepared  with  nitrate  of  silver  ;  and  the  intensity  of 
light  by  the  distance  at  which  fine  print,  placed  in  different  parts  of 
the  spectrum,  can  be  read. 

The  position  of  the  maximum  intensity,  for  the  calorific  rays,  varies 
with  the  nature  of  the  material  of  the  prism. 

On  the  right  of  the  spectrum  are  three  curved  lines,  C,  L,  and  H, 
which  are  called,  respectively,  the  curve  of  chemical  intensity,  the  curve 
of  luminous  intensity,  and  the  curve  of  thermal  intensity.  The  most 
prominent  point  of  each  curve  stands  opposite  to  that  part  of  the  spec- 
trum in  which  is  found  the  maximum  effect  of  the  property  which  the 
curve  represents.  From  these  points  the  intensity  diminishes  in  pro- 
portion as  the  curves  approach  the  straight  base-line,  until  the  curves 
and  base-line  meet,  opposite  to  which  points  the  effects  cease. 


Complementary  colors.  —  Any  two  colors  are  said  to  be 
complementary  to  each  other,  which,  by  their  union,  would  produce 
white  light.  If  all  the  colors  of  the  solar  spectrum,  except  any  one  of 
them,  be  reunited  by  means  of  a  double  convex  lens  (Fig.  54),  or  by  a 
second  prism,  the  resulting  color  will  be  complementary  to  the  color  left 
out,  as  it  only  lacks  this  color,  mixed  with  it,  to  produce  white  light. 
If  the  red,  for  example,  be  left  out,  the  recomposition  of  the  other 
colors  will  give  a  bluish-green  ;  hence  red  and  green  are  complement- 
ary. In  this  manner  it  is  found  that 

Red  is  complementary  to  .............  ,  .  Green. 

Violet  red          "  "  ...............  Yellow  green. 

Violet                 "  "  ...............  Yellow. 

Violet  blue        "  "  ...............   Orange  Yellow. 

Blue                   "  "  ...............   Orange. 

Greenish  blue   "  "  ...............  Reddish  Orange. 

Black                "  "  ...............  White. 


.  Analysis  of  colors  by  absorption.—  Although  the  colors 
of  the  prismatic  spectrum  cannot  be  further  divided  by  refraction,  yet 
any  of  these  colors  may  be  still  further  decomposed  by  transmission 
through  various  colored  glass;  by  which  means  it  has  been  found  that 
red,  yellow,  and  blue,  are  in  all  parts  of  the  spectrum  ;  and  that  any 
color  whatever  may  be  formed  by  suitably  combining  these  three. 
Hence  it  is  inferred,  that  there  are  really  only  three,  instead  of  seven, 


240 


OPTICS. 


primary  colors,  red,  yellow ',  and  blue ;  the  other  four  being  considered 
as  secondary  colors  of  the  spectrum. 


Figure  47. — Union  of  two  primary  colors  of  the 
spectrum,  to  produce  a  secondary  color. — Let  a  solar  ray  of 
light  be  dispersed  by  the  prism  P,  and  intercepted  by  a  screen,  A,  so 
perforated  as  to  allow  the  primary  colors  yelloiv  and  lilue  to  pass 
through  it,  and  fall  upon  the  two  prisms,  H  and  N,  by  which  the  rays 

FIG.  47. 


will  be  still  further  refracted,  but  not  dispersed.  If,  by  means  of  the 
double  convex  lens,  L,  they  be  converged  so  as  to  meet  on  the  screen  S, 
they  will  form  green. 

By  the  same  process  it  is  found  that  yellow  and  red  form  orange; 
red  and  blue  form  violet  and  indigo.  Still,  no  two  colors  alone  make  a 
third  color  in  the  solar  spectrum,  as  more  or  less  of  all  the  primary 

colors  are  necessary  to  form  either  of  the  four  secondary  colors. 

. 

4%  4-  Figure  48.— Composition  of  the  several  colors  of 
the  solar  spectrum. — The  colored  spaces  represent  the  relative 


FIG.  48. 


lengths  of  the  several  colors  of  the  spectrum.     The  three  curves,  E,  Y, 
and  B,  represent,  respectively,  the  distribution,  along  the  spectrum,  of 


OPTICS.  241 

the  three  primary  colors,  red,  yellow,  and  blue,  and  the  relative  amount 
of  each  of  these  necessary  to  produce  the  secondary  colors  of  the  spec- 
trum, orange,  green,  indigo,  and  violet. 

The  curved  line,  B,  shows  that  very  little  blue  is  found  at  the  red 
end  of  the  spectrum  ;  and  the  curved  line  R,  that  there  is  hardly  any 
red  at  the  blue  end  ;  while  the  curved  line  Y,  shows  that  but  a  small 
portion  of  yellow  is  found  at  either  end. 

The  spaces  between  any  two  of  the  vertical  dotted  lines,  lying  be- 
tween the  curved  lines  and  spectrum,  represent  the  relative  amount  of 
each  of  the  primary  colors  necessary  to  form  the  secondary  color  that 
lies  directly  below. 


.  Refraction  and  dispersion  of  the  solar  spectrum.  — 
If  a  glass  tube  or  a  plain  drin  king-glass,  or  any  glass  instrument  of 
similar  form,  be  held  in  the  path  of  the  colored  rays  of  the  spectrum,  in 
a  dark  room,  a  beautiful  system  of  colored  rings  will  be  produced, 
which  vary  in  form,  position,  and  color,  with  every  change  in  the 
position  or  form  of  the  interposed  glass.  The  great  variety  and  exquis- 
ite beauty  of  the  tints  and  hues  exhibit  the  infinite  resources  of  color 
in  the  sunbeam. 

Dark  Lines  in  Light. 

4^6.  Dark  lines  in  the  solar  spectrum  (Fig.  48).—  If  the 
spectrum  be  formed  from  a  narrow  line  of  light,  and  by  a  fine  flint- 
glass  prism,  and  viewed  through  a  telescope,  there  will  be  seen  cross- 
ing the  spectrum  a  large  number  of  dark  lines,  of  different  sizes  ; 
varying  in  number  from  600  to  2,000,  according  to  the  power  of  the 
telescope.  None  of  the  dark  lines  coincide  with  the  boundaries  of  the 
colored  spaces. 

The  position  of  some  of  the  largest  of  these  lines  is  shown  by  the 
diagram,  the  lines  being  drawn  in  their  relative  positions  lelow  instead 
of  across  the  spectrum. 

427.  Lines  in  light  vary  with  different  sources  of  light. 

-The  position  of  the  dark  lines  of  the  spectrum  is  invariable  when 
the  light  comes  from  the  sun,  whether  the  spectrum  be  formed  from 
direct  rays  or  from  rays  reflected  by  the  moon,  planets,  or  terrestrial 
objects.  When  the  spectrum  is  formed  from  light  of  a  star,  the  posi- 
tion and  number  of  the  lines  are  not  the  same  as  when  it  is  formed 
from  the  light  of  the  sun.  Their  position  and  number  are  not  the 
same  when  the  spectrum  is  formed  from  light  of  different  stars.  Elec- 
trical light  and  light  of  flames,  from  whatever  burning  body,  give 
bright  lines  instead  of  the  dark  lines. 

16 


242  OPTICS. 


428.  Fixed  lines  in  the  spectra  of  different  colored 

flames.  —  Salts  of  various  metals  impart  characteristic  colors  to  the 
flame  of  alcohol,  and  spectra  from  flames  thus  colored  possess  character- 
istic fixed  lines.  For  instance,  the  spectrum  of  a  soda-flame  is  charac- 
terized by  two  bright  lines  in  the  position  of  two  dark  lines  in  the  solar 
spectrum.  Flames  of  potash-salts  give  other  bright  lines  in  the  place 
of  certain  other  dark  lines. 

Experiments  with  such  and  various  other  flames  have  led   some 
philosophers  to  infer,  that  the  atmosphere  of  the  sun  contains  com- 
pounds of  sodium  and  potassium. 
• 

Colors  of  Bodies. 

429.  Color  of  opaque  bodies.  —  The  color  of  a  body  may  be 
temporary  or  permanent.     Temporary  colors  arise  from  some  modifica- 
tion of  light,  of  a  transient  character.     The  colors  of  a  rainbow,  for 
instance,  are  caused  by  refraction,  and  are  temporary. 

Respecting  permanent  colors  there  are  various  opinions.  Newton 
held  that  bodies  absorbed  some  of  the  rays  of  the  spectrum  and  re- 
flected the  remainder.  According  to  this  theory,  the  color  of  a  body 
would  be  that  resulting  from  a  mixture  of  the  reflected  rays.  For 
instance,  vermilion  was  supposed  to  reflect  only  the  red  rays,  while  it 
absorbed  all  the  other  rays.  All  bodies  placed  in  red  light  appear  red, 
in  blue  light,  blue,  and  so  on  for  other  colors. 

Arago  was  of  the  opinion  that  the  color  of  bodies  arose  from  light 
admitted  into  the  body  and  then  emitted  again,  undergoing  thereby 
certain  modifications.  According  to  this  theory,  the  color  would  de- 
pend upon  the  molecular  condition  of  the  body. 


Color  of  transparent  bodies.  —  All  transparent  bodies 
absorb  more  or  less  of  the  light  which  enters  them,  and  if  sufficiently 
thick,  must  appear  colored.  Their  color,  therefore,  is  due  to  that  part 
of  the  light  which  is  transmitted.  If,  for  example,  all  the  solar  rays, 
except  the  red  ones,  are  absorbed  by  a  medium,  it  will  appear  red  by 
transmitted  light.  Hence  water,  in  large  masses,  appears  greenish,  by 
absorbing  more  rays  of  the  other  colors  and  transmitting  more  of  this 
hue.  In  the  same  way  air  appears  blue,  and  hence  the  color  of  the  sky. 

• 

431.  Recomposition  of  light.  —  The  seven  colors  of  the  spec- 
trum may  be  reunited  so  as  to  produce  white  light.  This  can  be  done 
in  several  ways. 

1.  If  a  circular  disk  of  card-board  be  painted  in  sectors  with  the 
seven  colors  of  the  spectrum,  in  the  proportion  of  56°  red,  27°  orange, 
27°  yellow,  46°  green,  48°  blue,  47°  indigo,  109°  violet,  and  then  rapidly 


OPTICS. 


243 


revolved,  it  will  appear  to  be  painted  white;   illustrated  by  Fig.  52 
(p.  245).     In  this  case,  the  colors  are  mixed  in  the  eye. 

2.  If  the  spectrum  be  received  upon  a  concave  mirror,  it  will  be 
reflected  to  a  focus,  producing  white  light. 

3.  If  the  rays  of  the  solar  spectrum  be  passed  through  a  double  con- 
vex lens,  shown  by  Fig.  54  (p.  247),  they  will  be  reunited ;  and  if  a 
screen,  S,  be  held  at  the  focus  of  the  lens,  an  image  will  be  formed 
entirely  free  from  color. 

4.  If  light  be  decomposed  by  a  prism,  and  then  received  upon  a 
second  prism,  of  the  same  form  and  material,  having  its  refracting 
angle  reversed,  it  will  be  recomposed  and  emerge  as  white  light;  as 
shown  by  Fig.  4,  Chart  6  (p.  251).     The  incident  ray,  L,  will  emerge 
from  the  second  prism  in  the  direction  of  H ;  the  incident  and  emerg- 
ent rays  being  parallel,  as  represented  by  the  dotted  lines. 

Rainbows. 

432.  Figures  49  and  50.— Rainbows— primary  and  sec- 
ondary.— The  rainbow  is  a  semi-circular  band  or  arch,  composed  of 

FIGS.  49  AND  50. 


the  seven  different  colors,  seen  in  the  air  opposite  to  the  direction  of 
the  sun,  during  the  occurrence  of  rain  in  sunshine,  when  the  sun  is 
less  than  42°  above  the  horizon. 


244  OPTICS. 

. 

This  beautiful  phenomenon  in  nature  is  caused  by  reflection,  refrac- 
tion, dispersion,  and  interference  of  sunbeams  by  drops  of  rain. 

Sometimes  there  are  two  rainbows,  one  within  the  other ;  the  inner  one 
being  called  the  primary  bow,  and  the  outer  one  the  secondary  bow. 

Let  H  and  N  be  two  drops  of  rain  falling  through  air,  and  S  two 
rays  of  light,  one  of  which  falls  upon  N,  and  the  other  upon  H.  N 
belongs  to  the  primary  and  H  to  the  secondary  bow. 

The  ray  falling  upon  N  will  first  be  refracted  and  dispersed,  then 
internally  reflected  at  L,  and,  finally,  on  emerging  from  the  drop,  be 
again  refracted.  The  red  ray,  E,  falling  the  lowest,  -will  pass  to  the 
eye,  1,  while  the  other  rays  are  thrown  above  the  eye. 

The  ray  falling  upon  H,  will  first  be  refracted  and  dispersed,  then 
twice  internally  reflected,  at  L  and  T,  and,  finally,  on  emerging  from 
the  drop,  be  again  refracted.  The  violet,  in  this  case,  owing  to  the 
double  reflection,  falling  the  lowest,  will  also  pass  to  the  eye  1,  while 
all  the  others  will  be  thrown  above  the  eye.  A  person,  therefore, 
standing  in  this  position,  will  observe  a  red  ray  from  the  primary  and 
a  violet  ray  from  the  secondary  boiv  ;  while  the  eye  2  will  observe  the 
orange  ray  of  the  primary  bow,  and  the  indigo  ray  of  the  secondary 
bow;  and  so  on,  until  the  eye  7  will  take  in  the  violet  ray  of  the  pri- 
mary bow  and  the  red  ray  of  the  secondary  botv.  Hence  it  will  be  seen, 
that  by  placing  the  eye  in  seven  different  positions,  it  will  observe  all 
the  colors  of  the  rainbow  in  one  drop,  and  in  two  drops  all  the  colors 

coming  from  both  bows. 

FIG.  51. 


OPTICS.  -M5 

433.  Figure  51.— How  we  see  all  the  colors  of  the  rain- 
bow from  one  position. — It  has  just  been  shown  how,  from  differ- 
ent positions,  all  the  colors  of  the  rainbow  may  be  seen  in  one  drop. 
It  will  be  seen  by  this  diagram,  how,  by  a  series  of  drops,  all  the  colors 
can  be  seen  from  one  position. 

Let  SS  be  rays  of  the  sun  incident  upon  the  series,  or  constant  suc- 
cession of  drops  on  the  left,  and  it  will  be  seen  that  the  uppermost  drop 
will  send  the  red  ray  to  the  eye;  and  the  next  drop,  the  orange  ray; 
and  so  on,  to  the  lowermost  drop,  which  will  furnish  the  violet  ray. 

Hence,  as  two  persons  cannot  occupy  the  same  place  of  observation, 
it  is  evident  that,  although  different  Fro.  52. 

persons  observe  this  phenomenon  at 
the  same  time,  no  two  persons  behold 
exactly  the  same  rainbow. 

The  unequal  brilliancy  of  the  two 
bows  is  due  to  a  greater  loss  of  light 
in  the  secondary  than  in  the  primary 
bow,  caused  by  the  light  being  twice, 
instead  of  once,  reflected. 

Figure  52. — For  an  explanation 
of  this  diagram,  see  431. 

434.  Figure  53.— The  arch  of  the  rainbow.— It  is  not  so 

difficult  to  understand  the  refraction,  reflection,  and  dispersion  of  a  ray 
of  light  by  a  drop  of  water,  as  it  is  to  comprehend  the  construction  of 
the  arch  of  the  bow. 

Width  of  the  bozv.—By  referring  to  Fig.  51  (433),  it  will  be  seen  that 
the  angle  formed  by  the  incident  ray  and  the  reflected  or  red  ray  (pass- 
ing to  the  eye)  of  the  uppermost  drop,  is  larger  than  the  angle  formed 
by  the  incident  ray  and  the  reflected  or  violet  ray  (passing  to  the  eye) 
of  the  lowermost  drop. 

Now,  as  the  eye,  in  both  cases,  is  in  the  same  position,  and  the  sun's 
rays  are  parallel  to  each  other,  the  difference  in  the  size  of  the  two  an- 
gles is  owing  to  the  fact  that  the  reflected  ray  of  the  lowermost  drop, 
being  the  violet,  is  more  refracted  than  the  reflected  ray  of  the  upper- 
most drop,  which  is  the  red  ray. 

The  angle  formed  by  the  sunbeam  and  red  ray  being  42°  4',  and  that 
formed  by  the  sunbeam  and  violet  ray  being  40°  17',  their  difference 
will  be  1°  47',  which  is  the  width  of  the  primary  bow. 

The  width  of  the  secondary  bow  (the  light  being  twice  reflected)  is 
greater,  by  3°  10'.  In  this  bow  the  angle  formed  by  the  sunbeam  and 
red  ray  is  50°  57'.  As  the  red  ray  is  on  the  outside  of  the  primary 


246 


OPTICS. 


bow  and  on  the  inside  of  the  secondary  bow,  the  distance  between  the 
lows  will  be  the  difference  between  the  two  angles  formed  by  the  sun- 
beams and  the  two  red  rays,  that  is,  50°  57'  minus  42°  4'  equals  8°  53'. 
The  arch  of  the  bow  (Fig.  53).   Let  the  uppermost  drop  of  the  primary 

FIG.  53. 


bow,  and  the  lowermost  drop  of  the  secondary  bow,  represent  drops  re- 
flecting the  red  rays.  The  angles  formed  by  the  sunbeams,  S  and  L, 
and  these  red  rays,  as  above  shown,  are  respectively  42°  4'  and  50°  57', 
and  constant  or  invariable,  whatever  be  the  position  of  the  drops. 
Therefore,  although  the  air  on  the  one  side  is  filled  with  drops  of  rain, 
and  on  the  other  side  with  sunbeams,  and  every  drop  refracts,  reflects, 
and  disperses  the  light,  yet  the  observer,  in  any  one  position,  can  view 
only  those  rays  which  are  embraced  within  the  several  angles  as  above 
explained.  That  is,  if  a  spectator  stand  with  his  back  to  the  sun,  and 
a  straight  line  be  drawn  from  the  sun,  through  the  eye  to  the  shower 
of  rain,  it  will  also  pass  through  the  centre  of  the  bow  ;  and  the  obser- 
ver will  perceive  the  violet  of  the  primary  everywhere  40°  17'  from  this 
line ;  and  the  red  of  the  primary  42°  4';  and  the  red  of  the  secondary 


OPTICS. 


50°  57' ;  and  the  violet  of  the  secondary  54°  7'.  Hence,  if  the  angle 
of  the  sun's  elevation  above  the  horizon  exceeds  these  angles,  no  rain- 
bow can  be  seen ;  and  the  nearer  the  sun  is  to  the  horizon,  the  higher 
will  be  the  rainbow. 

The  purity  of  the  several  colors  in  the  rainbow  is  the  result  of  inter- 
ference, which  produces  dark  bands  for  each  particular  color,  giving  a 
clear  space  for  the  delineation  of  the  other  colors  of  the  rainbow  before 
the  first  color  is  repeated.  The  colors  are  most  clearly  defined  when 
the  drops  of  rain  are  uniform  in  size. 

For  a  further  explanation  of  the  rainbow,  or  the  effects  of  a  drop  of 
water  on  a  ray  of  light,  see  Fig.  1,  Chart  6  (435). 


FIG.  54. 


Figure  54. — Recomposition  of  Light  by  means  of  a  double 
convex  lens.     For  an  explanation  of  this  diagram,  see  431. 


248 


OPTICS. 


CHAPTER    XI. 


(CHART  NO.  6.) 
OPTICS,  CONTINUED, — OPTICAL  INSTBUMENTS. 

435.  Figure  1.— Effects  of  a  drop  of  water  upon  parallel 
rays  of  light,  further  explained. — Let  the  circle  represent  a 
drop  of  water,  and  S,  T,  H,  F,  parallel  rays  falling  upon  it.  The  ray 
F,  being  perpendicular  to  the  drop,  will  pass  straight  through  it,  as 
shown;  though  a  little  of  its  light  will  be  externally  reflected  back 
upon  itself  at  the  first  surface,  and  internally  at  the  second.  The  ray 
H  will  be  refracted  to  A,  where  it  will  be  reflected  to  the  opposite  sur- 

FIG.  1. 


face  and  then  refracted  in  the  direction  of  Y,  making  a  certain  angle 
with  its  original  direction  H.  As  the  distance  of  the  incident  ray  from 
the  centre  increases,  the  emergent  ray  will  make  a  greater  angle  with 
the  incident  ray,  until  a  distance  is  reached  where  the  angle  formed  by 
the  incident  and  emergent  rays  will  be  the  largest  possible,  as  shown  by 
the  heavy  incident  line  T,  and  its  emergent  ray  K  ;  for,  the  more  dis- 
tant ray  S  will  emerge  at  E,  nearer  parallel  to  itself  than  the  heavy  line 
T.  Hence,  any  incident  ray  on  either  side  of  T,  will  emerge  parallel  to 


OPTICS.  249 

some  other  emergent  ray,  whose  incident  ray  is  on  the  opposite  side  of 
T  ;  as  shown  by  the  two  finer  lines  drawn  close  to  the  heavy  line  T, 
which  emerge  in  the  direction  of  L,  parallel  to  each  other. 

Now,  as  the  emergent  ray  K  makes  the  greatest  possible  angle  with 
the  incident  rays,  it  follows  that  all  the  parallel  rays  which  enter  the 
drop  will  emerge  and  spread  over  and  be  limited  to  the  space  between 
the  rays  K  and  P,  having  the  greatest  intensity  near  the  direction  K, 
and  rapidly  diminishing  toward  Y,  until  they  fade  to  nothing. 

The  angle  of  greatest  deviation,  TAK,  for  red  rays,  is  42°  4',  and  for 
violet  rays,  40°  17',  as  previously  stated  (434). 

Since  there  are,  at  every  angle  between  K  and  F,  parallel  rays,  which 
have  traversed  different  paths,  and  so  unequal  distances,  through  the 
drop,  there  will  be  exhibited  all  the  phenomena  of  bright  and  dark 
bands,  produced  by  interference,  to  be  hereafter  explained. 

The  intersection  of  the  emergent  rays  will  form  a  caustic  curve  (391). 


.  Fogbows.  —  Hales*  —  Coronas.  —  Fogbows  differ  from 
rainbows  by  the  minuteness  of  the  globules  of  water  from  which  the 
reflection  takes  place. 

Halos  are  prismatic  rings  seen  around  the  sun  or  moon,  varying 
from  2°  to  46°  in  diameter,  caused  by  reflection  from  minute  crystals 
of  ice  floating  in  the  air. 

Coronas  are  formed  about  the  moon  by  reflection  from  the  external 
surface  of  aqueous  vapor,  the  light  thus  reflected  interfering  with 
direct  light  from  the  same  source. 


.  Figure  2.  —  Chromatic  aberration.  —  From  the  analo- 
gous action  of  prisms  and  lenses,  previously  shown  (411-12),  it  is  evi- 


FIG.  2. 


i 
I 


dent  that  white  light  will  be  dispersed  by  convex  lenses,  in  the  same 
manner  as  by  prisms,  producing  all  the  colors  of  the  spectrum. 


250  OPTICS. 

Let  F  be  an  object  situated  beyond  the  principal  focus  of  the  lens  L ; 
which,  having  the  same  effect  as  the  two  inscribed  prisms,  will  refract 
the  most  refrangible  rays,  which  are  the  violet,  to  a  focus  nearer  the 
lens,  than  the  least  refrangible,  which  are  the  red  rays.  Hence  there 
will  be  formed,  as  at  V,  a  violet  image  of  the  object,  F ;  and  a  red 
image,  beyond,  as  at  R;  while  images  of  the  other  colors  of  the  spec- 
trum will  be  formed  between  the  violet  and  red.  Though  the  image 
of  a  point  or  line  formed  at  V  is  violet,  yet  it  will  be  surrounded  by 
fringes  composed  of  all  the  colors  of  the  spectrum,  the  outer  border  of 
the  fringe  being  red.  If  the  lens  be  28  inches  focus,  the  distance  be- 
tween the  images,  V  and  E,  will  be  1  inch ;  if  28  feet,  it  will  be  1  foot. 

This  scattering  of  the  different  colored  foci,  which  occurs  with  the 
use  of  all  single  lenses,  formed  of  whatever  substance,  is  called  chro- 
matic aberration. 

Hence,  for  many  nice  optical  purposes,  as  for  the  telescope  and  mi- 
croscope, a  lens  or  any  combination  of  lenses,  formed  out  of  the  same 
glass,  is  almost  entirely  useless. 

438.  Figure  3. — Achromatic  combination  of  lenses. — To 

overcome  the  chromatic  aberration  of  lenses,  and  render  them  suitable 
for  such  optical  instruments  as  the  telescope,  microscope,  etc.,  has  led 
to  a  combination  of  lenses,  which  has  the  effect  of  neutralizing  their 
dispersive,  by  partly  destroying  their  refractive,  power. 

Such  lenses  consist  of  a  combination  of  two  or  more  lenses  of  diffe- 
rent shapes,  and  made  of  materials  of  unequal  dispersive  power ;  by 
which  images  can  be  produced  unattended  with  prismatic  phenomena. 

The  double  convex  crown-glass  lens  L  (acting  as  two  prisms,  base  to 

FIG.  3. 


base)  will  refract  the  parallel  lines,  HN,  to  its  principal  focus,  F,  at- 
tended with  the  prismatic  colors.      If,   however,  the  double  concave 


OPTICS.  251 

crown-glass  lens  T,  having  its  concavity  equal  to  the  convexity  of  L, 
be  placed  in  the  position  as  shown,  it  will  act  as  two  prisms,  apex  to 
apex,  and  render  the  converging  rays  parallel.  In  this  case  one  lens 
just  neutralizes  the  other  in  every  respect.  To  retain  a  part  of  the 
refractive  effect,  and,  at  the  same  time,  neutralize  the  prismatic  or  dis- 
persive eifect,  the  concave  lens,  T,  is  made  of  flint  glass,  which  material 
having  double  the  dispersive  power  of  crown-glass  (419),  may  be  made 
plano-concave,  instead  of  double  concave,  and  so  still  neutralize  the 
dispersive  effect  of  the  double  convex  crown-glass  lens,  L,  while  it  will 
neutralize  only  half  its  refractive  effect ;  therefore,  in  such  a  combina- 
tion, the  refractive  power  is  equal  to  a  single  plano-convex  lens  of 
crown-glass,  of  the  same  curvature  as  L.  Hence,  the  focus  of  the  com- 
bination will  be  at  the  point  E,  double  the  distance  from  the  lens  of  the 
focus  F,  and  free  of  prismatic  colors. 

In  forming  an  achromatic  combination  the  following  conditions  must 
obtain : 

1st.  It  must  be  composed  of  two  or  more  lenses,  formed  of  media 
having  different  dispersive  powers. 

2d.  One  of  the  lenses  must  be  concave  and  the  other  convex. 

3d.  The  two  lenses  must  have  focal  lengths  directly  proportional  to 
the  dispersive  powers  of  the  media  of  which  they  are  respectively 
composed. 

If  the  combination  is  made  of  crown  and  flint  glass,  the  focus  of 
the  croivn  should  be  to  that  of  the  flint  as  2  to  3,  or  rather,  in  ordi- 
nary cases  as  to  3  to  4  ;  since  most  specimens  of  flint-glass,  when  form- 
ed into  an  equal  prism  with  one  of  crown,  make  a  spectrum  whose 
length  compared  with  that  of  the  crown  is  as  4  to  3,  instead  of  3  to  2, 
as  heretofore  stated. 

FIG.  4. 


Figure  4.— Recomposition   of  light   by  means  of  reversed 
prisms.     For  an  explanation  of  this  diagram,  see  431. 


OPTICS. 


VISION. 


. — Figure  6.— The   camera  obscura.— The  camera  ob- 
ecura  as  the  name  implies,  is  a  dark  chamber,  and,  in  a  portable  form, 


FIG.  5, 


is  employed  to  take  pictures  of  objects.  In  its  simplest  construction, 
it  consists  of  a  dark  room,  provided  with  a  small  opening,  as  a  hole  in 
a  shutter,  on  one  side  of  the  room,  the  wall  on  the  opposite  side  serv- 
ing as  a  screen,  to  receive  the  image. 

The  principle  of  its  operation  is,  that  the  rays  which  form  the  image, 
coming  from  any  object,  as  the  tower  H,  must  converge  in  order  to 
pass  through  the  opening  T ;  hence,  all  rays,  except  the  axial,  neces- 
sarily cross  each  other  as  they  enter  the  box  or  chamber ;  and,  as  all 
other  rays  are  excluded,  an  inverted  image  will  be  formed,  as  repre- 
sented. 

To  render  the  camera  obscura  portable,  there  is  substituted,  in  place 
of  the  room  with  an  aperture  in  the  shutter,  a  wooden  box,  with  a 
small  hole  in  due  side,  with  a  ground-glass  or  paper  screen  on  the 
other,  upon  which  to  receive  and  trace  the  image. 

If  the  box  be  moved  toward  the  object,  the  image  increases  in  size, 
and  diminishes  if  it  be  moved  from  it.  If  it  be  held  at  a  given  dis- 
tance, and  the  opening  moved  up  or  down,  or  to  the  right  or  left,  as  if 
the  box  were  held  by  a  central  point  within,  the  image  will  remain  the 
same  size,  but  be  shifted  about  on  the  screen.  The  image  is  rendered 
more  distinct  with  a  small  hole  than  with  a  large  one,  since,  in  the  first 
case,  rays  from  any  particular  part  of  the  object  fall  on  the  correspond- 
ing part  of  the  image. 

The  images  are  the  same  whatever  be  the  shape  of  the  aperture,  pro- 
vided it  be  quite  small. 

The  images  thus  formed  are  not  sufficiently  distinct,  but  if  the  aper- 


OPTICS.  253 

ture  be  made  larger  and  provided  with  a  double  convex  lens,  the  pic- 
ture will  be  formed,  on  a  screen  placed  at  the  focal  distance,  which  will 
represent,  with  great  beauty  and  distinctness,  whatever  is  in  front  of  the 
lens,  all  the  objects  having  their  proper  relations  of  position,  light, 
shadow,  and  color. 

This  instrument  affords  aid  in  sketching  outlines  of  landscapes, 
buildings,  etc. ;  but  its  principal  importance  at  present  consists  in  its 
application  to  the  various  branches  of  Photography  (487). 

The  camera  obscura  will  be  alluded  to  again,  and  it  is  thus  far  ex- 
plained in  this  connection,  to  assist  in  explaining  the  construction  of 
the  eye  and  the  phenomena  of  vision. 

440.  The  eye  a  camera  obscura. — The  eye  is  a  self-adjusting 
camera  obscura ;  the  eyeball  being  the  dark    chamber  or  box ;  the  pu- 
pil, the  aperture;  the  retina,  the  screen;  the  contents  of  the  ball,  the 
lens.    Its  portability  is  evident,  as  we  always  have  two  of  them  with  us. 
Its  position  is  adjustable  by  the  motion  of  the  head  in  every  direction 
on  the  neck,  and  by  the  partial  rotation  of  the  ball  within  its  socket. 
The  size  of  the  pupil  or  aperture  adjusts  itself  to  the  intensity  of  the 
light ;  the  humors  and  lens  adjust  themselves  to  vary  the  focal  distance 
to  the  retina  or  screen,  according  to  the  distance  of  the  object,  and 
they  have  neither  the  fault  of  spherical  or  chromatic  aberration. 

By  these  various  and  wonderful  contrivances,  we  can  stand  in  one 
position,  and,  without  moving  the  body  below  the  neck,  so  set  and  ad- 
just this  natural  camera  obscura,  as  to  photograph  upon  the  retina  a 
perfect  image  of  every  object,  from  a  simple  speck  to  a  complicated 
landscape,  in  whatever  direction,  and  for  a  vast  distance.  Hence  the 
eye  may  be  considered  an  optical  instrument  embracing  every  per- 
fection. 

441.  Figure  6.— Method  of  adjusting  the  pupil  or  aper- 
ture of  the  eye. — The  circular  portion  of  the  figure  represents  the 
iris,  or  that  part  of  the 

eye  which  determines  its 
color,  as  black,  hazel, 
blue,  gray,  etc. ;  the  dark 
part  being  the  aperture 
or  pupil,  which  admits 
the  light. 

The  iris  is  provided 
with  two  sets  of  muscles, 
radiating  and  circular. 
The  radiating  muscles, 
represented  by  the  wavy 


254  OPTICS. 

radiating  lines,  will  contract  and  the  circular  ones  expand,  when  the 
intensity  of  the  light  is  insufficient,  and  so  enlarge  the  pupil,  that  more 
rays  may  enter.  The  circular  muscles,  represented  by  the  circles,  on 
the  contrary,  contract,  and  the  radiating  ones  expand,  when  the  inten- 
sity of  the  light  is  too  great,  and  thus  exclude  a  portion  of  the  light. 

The  variation  of  the  size  of  the  pupil  may  be  noticed  by  observing 
the  eye  at  different  distances  from  a  bright  light,  at  night.  Its  sudden 
contraction  gives  pain,  as  when  going  immediately  from  a  dark  to  a 
brilliantly  lighted  room,  or  on  suddenly  opening  the  eyes  in  the  morn- 
ing light.  The  pain  on  such  occasions  is  partly  due  to  the  effect  of  too 
much  light  on  the  retina,  before  the  pupil  can  sufficiently  contract  to 
exclude  it. 

The  owl  is  unable  to  see  by  daylight  because  he  cannot  contract  the 
pupil  sufficiently  to  prevent  the  blinding  effect  of  the  rays,  while  it  ad- 
mits sufficient  light  to  enable  him  to  see  in  the  night. 

The  pupil  in  man  is  round ;  in  the  feline  tribe  it  is  vertically  elon- 
gated ;  in  ruminating  animals  its  elongation  is  horizontal. 

44%-  Figure  7.— The  means  of  adjusting  and  holding 
the  eye  in  the  direction  of  the  object.— The  diagram  represents 
the  exterior  of  the  eyeball,  P  being  the  pupil,  and  FME,  muscles. 
These  and  other  muscles  are  attached,  at  one  end,  to  the  eyeball,  and 

at  the  other  end  to  the  back 
part  of  the  bony  socket.  By 
their  alternate  contraction 
and  expansion  the  eye  can 
be  turned  in  any  direction, 
necessary  to  direct  the  pupil 
toward  the  object  to  be 
viewed  ;  as  if  the  ball  were 
turned  about  a  central  point 
within  ;  while  they  serve 
also  to  firmly  hold  the  eye  in 
any  given  position. 

443.  Figure  8.— Structure  of  the  interior  of  the  eye.— 

The  figure  shows  a  horizontal  section  of  the  eye,  the  upper  part  repre- 
senting the  side  toward  the  nose. 

The  principal  parts  of  the  eye,  not  already  described,  are  the  sclerotic 
coat,  cornea,  cJioroid  coat,  retina,  optic  nerve,  crystalline  lens,  aqueous 
humor,  and  vitreous  humor. 

1.  The  sclerotic  coat  is  the  outer  covering,  being  a  strong,  thick, 
opaque  membrane ;  that  which  is  called  the  white  of  the  eye  being  a 


OPTICS. 


255 


part  of  it.     This  membrane  has  a  posterior,  sieve-like  opening,  T,  for 
the  transmission  of  the  fibres  of  the  optic  nerve. 

2.  The  cornea    is  a  transparent  membrane  which  is  joined  to  the 

FIG.  8. 


edges  of  an  opening  in  the  anterior  part  of  the  sclerotic  coat,  through 
which  the  light  passes  to  the  eye ;  and  holds  the  same  relation  to  the 
sclerotic  covering  that  the  crystal  of  a  watch  does  to  the  case.  The 
cornea  is  more  oval  than  other  parts  of  the  ball,  as  represented  in  the 
diagram. 

3.  The  choroid  coat  is  a  vascular  membrane  lining  the  sclerotic  coat, 
and  is  covered  internally  with  the  pigmentum  nigrum,  a  dark  pigment 
which  darkens  the  chamber  and  prevents  internal  reflection  of  light. 

To  the  edges  of  an  opening  in  the  front  part  of  this  membrane  is 
joined  the  outer  edge  of  the  iris,  RR. 

4.  The  retina  and  optic  nerve.    The  retina  is  the  third  or  inner  mem- 
brane of  the  eye,  and  consists  of  an  expansion  or  spreading  out  of  the 
optic  nerve,  into  millions  of  fine  fibres,  forming  a  whitish,  delicate, 
lining  membrane  of  nerve-substance,  upon  which  the  images  of  exter- 
nal objects  are  formed. 

5.  The  crystalline  lens,  L,  is  a  transparent  body,  of  the  consistence 
of  gristle,  placed  just  behind  the  iris,  and  is  enveloped  in  a  transparent 
membrane  or  capsule,  which  adheres  by  its  borders  to  the  ciliary  pro- 
cess, SS.     The  posterior  surface  of  the  crystalline  lens  is  more  convex 
than  the  anterior,  as  shown.     This  lens  is  made  up  of  serrated  fibres, 
arranged  in  layers,  which  increase  in  density  from  the  circumference 
to  the  centre  of  the  lens. 

6.  The  aqueous  humor  is  a  transparent  liquid  which  fills  the  space 
between  the  cornea  in  front  and  the  crystalline  lens,  L.     In  this  liquid 
freely  floats  the  annular  curtain  or  iris,  RR;  which  divides  this  space 
into  the  anterior  chamber  (between  the  cornea  and  the  iris)  and  the 


256  OPTICS. 

posterior  chamber  (between  the  iris  and  crystalline  lens).     These  two 
chambers  communicate  freely  with  each  other  through  the  pupil,  P. 

7.  The  vitreous  humor  is  a  transparent,  gelatinous  fluid,  nearly  filling 
all  the  posterior  compartment  of  the  eye,  which  includes  all  the  space 
behind  the  crystalline  lens.  This  humor  is  enclosed  in  delicate  cellu- 
lar tissue. 


444-  ^ke  lachrymal  or  tear  gland,  and  eyelid.  —  The  eye 

being  necessarily  sensitive,  in  order  to  be  susceptible  to  the  delicate 
impressions  of  light,  requires  to  be  kept  moist,  clean,  and  free  from 
dust,  and  protected  from  the  air  and  light  while  we  sleep. 

These  requirements  are  admirably  provided  for  by  the  lachrymal 
gland  and  eyelid.  These,  taken  together,  may  be  called  a  delicate 
washing  apparatus,  ever  at  work,  during  our  waking  hours,  moisten- 
ing and  cleansing  the  eye  every  time  we  wink.  The  gland  may  be 
likened  to  a  sponge,  concealed  and  situated  just  above  the  eye,  which 
gathers  tear-fluid  from  the  blood-vessels  and  passes  it  down  to  the  eye  ; 
where  the  lid,  acting  as  a  soft,  suitable  cloth,  wipes  or  washes  the  nat- 
ural glass  of  the  eye,  and  coats  it  with  an  essential  film  of  moisture. 
Without  winking,  vision  soon  becomes  indistinct. 

445.  Figure  9.—  Adjustability  of  the  eye  to  different 
distances.  —  That  the  crystalline  lens  must  adjust  itself,  in  order  to 
vary  its  focal  distance,  and  so  produce  on  the  retina  dis- 
tinct images  of  objects  situated  at  different  distances,  is 
evident  from  the  laws  previously  explained,  relating  to 
lenses. 

This  variation  in  the  convexity  of  the  crystalline  lens 
is  accomplished  by  means  of  its  elasticity,  and  the  attach- 
ment of  its  membrane  or  capsule  to  the  ciliary  process, 
SS,  Fig.  8. 

Let  L  represent  the  lens  within  its  capsule  or  bag.  If 
the  case  or  bag  be  fastened  to  the  short  parallel  lines, 
and  these  lines  be  separated,  the  lens  will  be  compressed 
and  flattened  (shown  by  the  dotted  lines),  as  if  it  were 
made  of  some  soft  elastic  substance,  like  rubber.  If  then 
these  points  of  attachment  be  brought  nearer  together, 
the  lens  will  restore  itself  to  its  former  convexity.  The  distance  of  the 
lens  from  the  retina,  also,  may  be  changed. 

446-  Optical  axis.  —  The  principal  axis  of  the  eye,  called  the 
optical  axis,  is  a  straight  line  passing  through  the  eye  in  such  a  direc- 
tion that  the  organ  is  symmetrical  on  all  sides  of  it  ;  which  is  a  right 


OPTICS.  25? 

line  passing  through  the  centre  of  the  cornea,  pupil,  and  crystalline 
lens.  Lines  drawn  near  the  optic  axis,  which  are  sensibly  right  lines, 
are  secondary  axes.  Objects  are  seen  most  distinctly  in  the  principal 
optic  axis. 

44?  '  .  Optic  angle.  —  The  angle  formed  by  drawing  straight  lines 
from  the  two  eyes  to  the  object,  is  called  the  optic  angle,  or  the  'binoc- 
ular parallax. 

This  angle,  and  difference  of  direction,  will  be  appreciated  by  look- 
ing at  an  object  first  with  one  eye  and  then  with  the  other,  without 
moving  the  head,  which  will  cause  the  object  to  apparently  change  its 
position. 


.  Angle  of  vision  (Figure  8).  —  The  angle  formed  at  the 
eye,  by  lines  drawn  from  the  extremities  of  an  object  to  where  they 
cross  in  the  eye,  is  called  the  visual  angle  or  angle  of  vision. 

This  angle  bears  a  proportion  directly  to  the  linear  magnitude,  and 
inversely  to  the  distance  of  the  object. 

1.  Let  AB  be  an  object,  and  the  lines  AN  and  BN  will  intersect  at 
the  eye,  forming  a  certain  angle;  and  an  image,  NN",  of  the  object,  of 
a  certain  size,  will  be  formed  on  the  retina.     If  the  object,  AB,  were 
made  double  its  length,  the  visual  angle  and  the  image  would  be  twice 
as  large. 

2.  If  the  object,  AB,  be  placed  at  EF,  half  as  far  from  the  eye,  the 
visual  angle  and  image  will  be  twice  as  large,  as  shown  by  the  dotted 
lines. 

The  reason  of  this  is,  that  the  diverging  lines  depart  from  each  other 
in  proportion  as  they  are  extended  from  the  point  of  their  intersection, 
as  shown  by  the  objects  1,  2,  and  3. 

As  the  superficial  magnitude  of  an  object  is  as  the  square  of  the 
lineal  magnitude,  the  apparent  superficial  magnitude  of  an  object  will 
be  inversely  as  the  square  of  the  distance. 

TJie  smallest  visual  angle  under  which  an  object  can  be  seen,  with 
the  naked  eye,  is  about  twelve  seconds. 

449.  Inversion  of  images  formed  in  the  eye.  —  The  camera 
obscura  (439)  and  the  structure  of  the  eye  are  sufficient  proof  of  the 
inversion  of  images  on  the  retina  ;  but,  for  ocular  proof,  take  the  eye 
of  an  ox,  cut  away  the  posterior  part  of  the  sclerotic  and  choroid  coats  ; 
fix  the  eye  in  an  opening  in  the  shutter  of  a  dark  room,  look  at  it  with 
a  magnifying  glass,  and  external  objects  will  be  seen  beautifully  delin- 
eated in  an  inverted  position  on  the  retina. 

17 


258  OPTICS. 

450.  —  Why  we  see  objects  erect,  their  images  being  in- 
verted, is  explained  in  different  ways  by  different  philosophers  ;  but 
probably  it  is  because,  the  image  always  being  inverted,  the  mind,  by 
unconscious  training,  is  habituated  to  it;  learning  from  the  beginning 
to  refer  the  impression  it  receives  to  the  upright  position  of  the  object. 

451.  —  The  brightness  of  the  ocular  image.  —  The  inten- 
sity of  light  diminishes  as  the  square  of   the  distance  it  travels  in- 
creases; see  Fig.  50  (529).     Hence,  the  brightness  of  an  object,  by  this 
law,  would  be  inversely  as  the  square  of  the  distance.     The  apparent 
superficial  magnitude  of  an  object  also  diminishes  as  the  square  of  the 
distance  increases  (447).     Hence,  as   the   intensity  of  the  light  (or 
brightness  of  the  object)  will  be  increased  by  the  apparent  diminu- 
tion of  surface  over  which  it  is  spread,  in  the  same  ratio  that  its  in- 
tensity will  diminish  by  the  increase  of  distance,  it  follows  that 

The  apparent  brightness  of  an  object,  and  consequently  of  its  image, 
ivill  remain  constant,  whatever  may  be  the  distance  of  the  object. 


.   Figure    10.  —Indistinct    vision.  —  If  an  object,  F,  be 
brought  too  near  the  eye,  say  within  an  inch  or  two,  its  image  becomes 

FIG.  10. 


confused  and  indistinct,  because  the  rays,  flowing  from  it,  fall  too 
divergent  on  the  crystalline  lens  to  be  refracted  to  a  focus  on  the 
retina. 

If  we  could  see  objects  distinctly  when  placed  quite  near  the  eye,  we 
should  be  able  to  examine  things  that  are  now  invisible  at  the  limit  of 
distinct  vision  (455),  since  the  visual  angle  (448)  would  then  be  in- 
creased, and  consequently,  the  image  on  the  retina  enlarged,  in  pro- 
portion as  objects  were  brought  near  the  eye. 

Sufficiency  of  illumination. — In  order  to  have  distinct  vision, 
the  image  must  not  only  be  well  defined  on  the  retina,  but  it  must  not 
be  so  faint  as  to  produce  no  sensation,  nor  so  intensely  brilliant  as  to 


OPTICS.  259 

dazzle  the  eye,  which  produces  pain,  and  consequently  destroys  dis- 
tinctness of  vision. 

453.  Figure  11.  —  How  to  see  objects  close  to  the  eye.  — 

The  images  of  objects  held   close   to  the  eye   may  be   rendered  dis- 
tinct by  intercepting  the  more  divergent  rays,  FIG.  11. 

and  thus  preventing  them  from  entering  the  eye. 
Let  the  object,  P,  be  a  pin-head,  brought  close 
to  the  eye.  Interpose  a  piece  of  paper,  sufficiently 
large  to  shut  off  all  the  light  that  would  fall 
upon  the  eye,  and  admit  only  such  rays  as  will 
pass  through  a  pin-hole  in  the  paper,  as  repre- 
sented. .These  few  rays,  being  sensibly  parallel, 
will  be  converged,  and  form,  on  the  retina,  not 
only  a  distinct  but  an  enlarged  image.  The 
brightness  of  the  image  will  be  diminished,  owing 
to  the  pin-hole  being  smaller  than  the  pupil. 


*  Figure  12.—  Brilliancy  of  vision  is  dependent  on  the 
number  of  rays  that  enter  the  eye,  that  can  be  brought  to  a  focus  on 
the  retina. 

If  the  object,  F,  be  a  pin-head,  and  L,  a  small  double  convex  lens, 
the  eye  will  receive  all  the  rays  that  would  diverge  between  the  two 

FIG.  12. 


dotted  lines ;   rendering  the  vision  more  distinct  than  if  only  those 
rays  were  received  which  would  diverge  to  the  eye  without  the  lens. 

This  is  the  whole  theory  of  the  single  microscope,  the  word  meaning, 
to  view  small  things. 

455.  Limit  of  distinct  vision. — Although  we  see  objects  at 
both  great  and  small  distances,  most  persons,  when  they  wish  to  see 
the  minute  structure  of  an  object  clearly,  place  it  from  six  to  ten  inches 
from  the  eye.  This  point  is  called  the  limit  of  distinct  vision. 


260  OPTICS. 

456.  Figure  13.— Visual  rays  must  be  nearly  parallel.— 

When  the  eye  is  adjusted  to  view  near  objects,  the  diameter  of  the 
pupil  being  only  about  one-tenth  of  an  inch,  and  the  limit  of  vision 
from  six  to  ten  inches,  it  will  be  found  that  the  cone  of  divergent  rays 
from  a  single  point  will  be  included  within  an  angle  of  from  one  to  a 
little  more  than  one-half  a  degree.  Therefore,  the  rays  of  vision  differ 
but  slightly  from  parallel  rays.  While,  for  all  objects  more  remote,  the 
rays  may  be  considered  as  parallel. 

Hence,  distinct  vision  is  obtained  only  by  rays  that  are  sensibly 
parallel  or  very  slightly  divergent. 

This  is  illustrated  by  the  diagram.  From  the  extremities,  as  well  as 
from  all  other  points  of  the  object,  L,  rays  diverge  in  every  direction ; 

FIG.  13. 


but  the  image,  T,  of  the  point,  L,  on  the  retina,  is  formed  by  the  few 
nearly  parallel  rays  that  pass  close  to  the  secondary  axis,  LT ;  and 
what  is  true  of  the  point  L  is  true  of  all  other  points  of  the  object. 

457 .  Size  of  the  image  on  the  retina. — The  actual  size  of  the 
image  on  the  retina,  capable  of  exciting  sensation  and  producing  vision, 
may  be  exceedingly  small.   For  example,  a  human  hair  can  be  seen,  with 
the  naked  eye,  at  a  distance  of  twenty  or  thirty  feet,  yet  the  image  is 
many  times  smaller  than  the  object. 

It  has  been  estimated  that  the  image  on  the  retina,  of  a  man  seen  at 
the  distance  of  a  mile,  is  not  more  than  one  five-thousandth  part  of  an 
inch  in  length. 

458.  Figure   14. — Near-sightedness  and  long-sighted- 
ness.— Persons  who  see  objects  at  very  short  distances,  say  less  than 
about  six  inches,  are  called  near-sighted  ;  while  those  who  see  objects 
distinctly  only  at  a  greater  distance  than  about  twelve  inches,  are  said 
to  be  long-sighted. 


OPTICS. 


261 


Near-sightedness  is  caused,  in  some  cases,  by  too  much  curvature  of 
the  cornea  and  crystalline  lens,  by  which  the  rays  of  light,  that  form 
the  image,  are  brought  to  a  focus  before  they  reach  the  retina,  as  shown 
by  the  image  T,  of  the  object  A,  which  falls  short  of  the  retina.  The 
object  will  be  seen,  but  not  distinctly.  To  obtain  distinct  vision,  there- 
fore, the  object  must  be  brought  nearer  to  the  eye,  or  concave  spectacles 
employed,  either  of  which  means  will  cause  the  rays  to  enter  the  eye 
with  a  greater  degree  of  divergence,  and  so,  by  increasing  the  focal 
distance,  throw  the  image  back  upon  the  retina. 

FIG.  H. 


Long-sightedness,  on  the  contrary,  is  caused  by  too  little  curvature  of 
the  cornea  and  crystalline  lens,  which  throws  the  image  S,  of  the  object 
A,  beyond  the  retina.  This  defect,  therefore,  is  corrected  by  holding 
the  object  at  a  greater  distance  from  the  eye,  or  by  employing  convex 
spectacles,  either  of  which  means  will  render  the  rays  less  divergent, 
and,  thus  shortening  the  focal  distance,  will  bring  the  image  within  the 
eye  and  throw  it  upon  the  retina. 

FIG.  15. 


459.  Figure  15. -Short-sightedness  and  long-sighted- 
ness caused  by  defective  forms  of  the  eyeball.— Although 


262  OPTICS. 

the  cornea  and  crystalline  lens  have  the  proper  curvature,  and  all  parts 
of  the  eye  possess  their  usual  powers  of  adjustment,  yet  short-sighted- 
ness may  result  from  an  elongation  of  the  eyeball,  which  would  locate 
the  retina  beyond  the  focal  distance,  and  thus  out  of  the  reach  of  the 
image. 

Long-sightedness  may  be  caused  by  the  shortening  of  the  diameter 
of  the  eyeball  in  the  direction  of  the  optical  axis ;  which  would  bring 
the  retina  between  the  crystalline  lens  and  its  focal  distance,  as  shown 
in  the  diagram. 

In  case  of  elongation,  as  shown  by  the  exterior  dotted  line  N,  the 
image,  L,  would  fall  short  of  the  retina ;  and  in  case  of  shortening,  as 
shown  by  the  interior  dotted  line,  TH,  the  image,  L,  would  fall  beyond 
the  retina. 

460.  Long-sightedness  of  old  people  is  due  principally  to 
the  loss  of  convexity  and  elasticity  of  the  crystalline  lens,  and  to  more 
or  less  diminution  of  curvature  of  the  cornea,  and  a  partial  absorption 

of  the  humors  of  the  eye. 

i 

461.  Conditions  of  distinct  vision  are,  1st.  That  an  object  be 
situated  at  such  a  distance  as  to  form  an  image  on  the  retina. 

2d.  That  the  image  be  of  some  appreciable  magnitude. 

3d.  That  the  object  be  sufficiently  illuminated  to  produce  a  distinct 
impression  upon  the  retina. 

An  image  may  be  so  faint  as  to  produce  no  sensation,  or  it  may  be 
so  intensely  brilliant  as  to  dazzle  the  eye,  destroy  the  distinctness  of 
vision,  and  produce  pain. 

462.  Sensibility  of  the  retina  is  diminished  by  long  exposure 
of  the  eye  to  intense  light,  and  increased  by  remaining  a  long  time  in 
feeble  light  or  in  the  dark  (441). 

That  that  part  of  the  retina,  receiving  a  very  bright  image,  will  be 
temporarily  insensible,  is  shown  by  turning  the  eye  directly  from  a 
bright  to  a  dim  object,  when  a  dark  spot  will  be  seen.  If  the  bright 
object  be  of  one  color,  the  part  of  the  retina  on  which  the  image  foils 
becomes  insensible  to  rays  of  that  color,  but  not  to  rays  of  other  colors. 

This  explains  the  appearance  of  complementary  colors  (421).  For  ex- 
ample, a  bright  red  image  will  blind  the  retina  to  red  light,  but  leave  it 
sensitive  to  the  remaining  colors  which  make  up  white  light;  and  when 
red  is  taken  from  white  light,  the  combination  of  the  other  colors  gives 
a  greenish  hue.  Hence,  on  turning  the  eye  from  a  bright  red  object, 
other  objects,  for  a  moment,  will  appear  greenish :  conversely,  if  the 
bright  object  be  green,  other  objects  will  appear  red. 


OPTICS.  263 

463*  Color-blindness.  —  Some  persons  are  unable  to  distinguish 
colors  at  all  ;  and  others  but  indifferently  ;  while  some  can  detect  the 
slightest  difference  in  shades  of  the  same  color.  Some  confound  red 
and  green,  and  can  distinguish  other  colors.  Persons  who  cannot  see 
the  difference  in  the  colors  of  the  spectrum,  or  cannot  distinguish  be- 
tween any  two  of  the  simple  colors,  are  said  to  be  color-blind. 


-  Effect  of  different  colors  on  vision.—  The  distance  at 
which  an  object  can  be  seen  varies  with  its  color  and  the  amount  of 
illumination.  A  white  object,  illuminated  by  the  sun,  can  be  seen  at  a 
distance  equal  to  17,250  times  its  own  diameter;  a  red  object,  about 
half  as  far;  and  a  blue  object  at  a  distance  somewhat  less  than  a  red. 
Objects  can  be  seen  about  twice  as  far  when  illuminated  by  direct  rays 
of  the  sun,  as  when  illuminated  by  ordinary  daylight. 

465.  Effects  of  background.  —  Irradiation.  —  When  a  white 
object  is  seen  against  a  black  ground,  it  appears  larger  than  it  really  is  ; 
while  a  black  object  on  a  white  ground,  appears  smaller  than  it  really 
is.    This  effect  is  called  irradiation. 

This  is  caused  by  the  impression,  produced  by  the  light-colored  object 
on  the  retina,  extending  beyond  the  outline  of  the  image.  It  bears  the 
same  relation  to  the  space  occupied  by  the  image,  as  the  duration  cf  the 
impression  does  to  the  duration  of  the  image. 

466.  Estimation  of  distance  and  magnitude  of  objects. 
—The  appreciation  of  the  distance  and  magnitude  of  objects  depends 

upon  the  visual  angle,  optic  angle,  comparison  with  familiar  objects, 
and  distinctness  or  dimness  of  the  image,  caused  by  intervening  air  or 
vapor. 

The  visual  angle  of  an  object,  as  previously  shown  (448),  varying 
with  the  distance,  can  afford  no  evidence  of  the  size  of  an  object,  unless 
we  appreciate  its  distance.  We  must,  therefore,  know  the  distance  of  a 
body  in  order  to  estimate  its  size.  By  knowing  its  distance  we  instinc- 
tively appreciate  its  size.  A  chair,  for  example,  at  the  opposite  side  of 
the  room,  has  a  visual  angle  only  half  as  large  as  when  at  half  the  dis- 
tance, yet  we  cannot  make  it  seem  any  smaller  in  one  part  of  the  room 
than  another,  if  we  try.  But  if  we  are,  in  any  way,  deceived  as  to  the 
distance  of  an  object,  we  are  also  deceived  as  to  its  size. 

One  of  the  means  by  which  we  judge  of  the  distance  of  an  object, 
is  by  knowing  its  size.  Being  familiar  with  the  size  of  many  bodies, 
as  men,  animals,  trees,  etc.,  the  visual  angles  under  which  they  are  seen 
enable  us  to  estimate  their  distance;  and,  knowing  their  distance,  we 
instinctively  estimate  the  magnitude  of  adjacent  objects,  with  whose 
magnitude  we  are  not  familiar. 


264  OPTICS. 

This  is  the  reason  why  the  moon  appears  larger  near  the  horizon 
than  overhead.  When  near  the  horizon  it  seems  further  off,  because  it 
is  heyond  all  other  objects,  and  so  we  judge  it  is  larger  than  when 
it  is  in  the  zenith,  where  there  are  no  intervening  objects  to  make  it 
appear  equally  distant. 

We  also  judge  of  the  distance  of  an  object  by  the  distinctness  with 
which  we  see  it.  The  brighter  it  is  the  nearer  it  seems.  It  is  for  this 
reason  that  objects  seem  larger  in  a  fog.  Their  indistinctness  im- 
presses us  that  they  are  far  off,  and  hence,  we  judge  they  are  larger 
than  they  are.  It  is  for  this  reason,  too,  that  distant  objects  seem  less 
distant  in  very  clear  atmosphere. 

Infants  reach  out  to  grasp  the  blaze  of  a  candle  which  is  many  feet 
from  them ;  showing  that,  without  experience  of  touch,  they  have  no 
notion  of  distance. 

The  optic  angle,  or  binocular  parallax  (447),  is  an  essential  means  in 
appreciating  distance.  This  angle  increases  or  diminishes  inversely  as 
the  distance.  The  effort  we  make  to  turn  the  eyes  inward,  to  vary  the 
optic  angle,  or  converge  the  optic  axes  of  the  two  eyes  upon  the  object, 
gives  us  an  idea  of  its  distance. 

467.  Why,  with   two    eyes,  we  see   objects   single.— 

Though  an  image  of  an  object  is  formed  in  both  eyes,  yet  we  see  but 
the  one  object.  This  is  accounted  for  by  the  bifurcation  of  the  optic 
nerves.  That  is,  the  optic  nerve  from  the  right  lobe  of  the  brain  sends 
a  portion  of  its  fibres  to  each  eye,  and  also  sends  some  branches  across 
and  backward  to  the  left  lobe  of  the  brain  ;  and  a  portion  of  the  optic 
nerve  from  the  right  eye,  instead  of  proceeding  to  the  brain,  curves 
around  to  the  optic  nerve  and  retina  of  the  left  eye.  The  optic  nerve 
of  the  left  side  is  related  to  that  of  the  right  side  in  the  same  manner. 

In  this  way  a  perfect  sympathy  is  established  between  the  two  eyes, 
the  inner  side  of  one  corresponding  to  the  outer  side  of  the  other.  As 
the  images  are  always  formed  with  their  centres  at  the  centres  of  the 
eyes,  the  right  and  left  parts  of  the  images  will  be  on  corresponding 
parts  of  the  eyes,  and,  therefore,  they  will  appear  as  one. 

By  pressing  the  finger  upon  the  eyeball,  the  images  will  not  fall  upon 
corresponding  parts  of  the  two  retinae,  and  the  object  is  seen  double. 

468.  Double  vision. — Both  eyes  being  fixed  steadily  upon  one 
object,  any  other  object  seen  at  the  same  time  will  be  seen  double. 

Fix  both  eyes  upon  any  near  object,  and  a  pencil,  held  between  the 
eyes  and  the  object,  will  appear  double. 

Any  cause,  as  drunkenness,  disease  of  the  nerves,  etc.,  which  prevents 
the  eyes  from  being  steadily  fixed  upon  the  same  object,  will  cause 
double  vision. 


OPTICS.  265 

469.  Binocular  vision. — Though  a  picture  of  an  object  is  formed 
on  the  retina  of  each  eye,  yet  the  two  pictures,  notwithstanding  they 
are  formed  from  the  same  object,  are  not  precisely  alike.     This  is 
because  the  object  is  not  observed  from  the  same  point  of  view  by 
both  eyes. 

If  a  thin  book,  for  example,  be  held  up  edgeways  to  the  centre  of 
and  a  few  inches  from  the  face,  one  eye  will  see  one  side  of  the  book 
and  the  other  eye  the  other  side. 

If  an  oval  object,  like  a  bottle,  be  held  before  the  face,  both  eyes  will 
see  some  portion  of  it,  while  each  eye  will  see  some  parts  of  it  that  the 
other  cannot  see ;  so  that  we  partially  see  around  the  bottle. 

While  the  mind  is  impressed  with  the  idea  that  there  is  but  one 
object,  yet  the  judgment  naturally  determines  the  object  to  be  a  pro- 
jecting body  (see  495). 

470.  Duration  of  impression  upon  the  retina. — The  im- 
pression made  by  light  on  the  retina  does  not  cease  instantly,  on 
removing  the  light,  but  lasts  for  an  eighth  of  a  second  or  more. 

A  lighted  stick,  as  every  one  has  observed,  whirled  rapidly  around  a 
circle,  appears  like  a  ring  of  fire.  The  rapidity  of  revolution  required 
to  produce  this  impression  is  one-third  of  a  second  in  a  dark  room,  and 
one-sixth  of  a  second  in  the  daylight.  It  is  owing  to  the  continuation 
of  the  impression  on  the  retina  that  the  seven  simple  colors,  revolved 
on  a  disk  (431),  produce  white  light;  and  that  the  spokes  of  a  rapidly 
revolving  wheel  cannot  be  distinguished. 

Winking  does  not  interfere  with  distinct  vision,  because  the  act  of 
winking  requires  less  time  than  is  needed  to  remove  the  impression 
from  the  retina. 

471.  Optic  toys.— rlt  is  upon  the  principle  that  impressions  re- 
main on  the  retina  for  a  sensible  length  of  time,  after  the  object  has 
changed  its  place,  that  optical  toys  are  constructed,  and  pyrotechnic 
exhibitions  owe  their  effect. 

472.  Time  required  to  produce  visual  impressions.— 

If  an  object  moves  across  our  vision  with  great  velocity,  as  a  projected 
cannon-ball  or  rifle-ball,  its  image  does  not  remain  on  the  retina  long 
enough  to  produce  any  impression. 

Motions  describing  less  than  one  minute  of  arc  in  a  second  of  time 
are  not  appreciable  to  us.  Hence,  we  cannot  perceive  the  movement 
of  the  hour-hand  of  a  clock,  or  the  motions  of  the  heavenly  bodies. 

473.  Sensations  of  light   may   be    excited   by  other 
causes. — The  sensation  of  light  may  be  excited  by  anything  which 


266  OPTICS. 

can  excite  the  optic  nerve.  An  electric  shock  sent  through  the  eye 
produces  an  apparent  flash  of  light.  If  a  piece  of  zinc  be  placed  in 
the  mouth  and  one  end  of  a  silver  pencil  held  in  the  corner  of  the  eye, 
a  flash  will  be  experienced  when  the  silver  and  zinc  are  brought  in 
contact.  Pressing  the  eyeballs  with  the  fingers  produces  a  luminous 
image ;  and  so  will  a  blow  on  the  head  enable  us  to  "  see  stars." 

OPTICAL     INSTRUMENTS. 

474-  Variety  and  principal  uses  of  optical  instru- 
ments.— There  are  many  kinds  of  optical  instruments,  varying  in 
construction  and  magnitude,  from  simple  eye-glasses  or  spectacles  to 
telescopes  weighing  many  tons,  and  costing  hundreds  of  thousands  of 
dollars. 

Though  the  unaided  eye  extends  its  limits  far  and  wide,  beyond  the 
reach  of  our  other  senses,  picturing  on  the  brain  an  infinite  variety  of 
objects,  yet  our  unassisted  power  of  vision  is  limited,  in  its  observa- 
tions of  the  vast  field  of  nature,  to  a  mere  speck,  compared  with  the 
scope  of  our  senses  aided  by  various  optical  instruments. 

The  microscope  (the  word  meaning  to  view  small  things)  has  made 
us  acquainted  with  a  world,  which,  though  too  minute  to  be  seen  with 
ordinary  eyes,  is  filled  with  greater  curiosities  and  wonders,  and  with 
more  important  operations,  than  all  we  can  see  by  direct  observation. 

The  telescope  (the  word  signifying  to  see  far  off)  has  made  it  possible 
for  us  to  bring  to  view  countless  worlds,  which,  though  of  immense 
magnitude,  were  beyond  the  reach  of  our  vision. 

With  the  aid  of  the  camera,  instead  of  painting  pictures  we  print 
them  with  rays  of  light. 

With  simple  lenses,  or  eye-glasses,  the  skill  of  the  artist  is  increased, 
and  the  dim  sight  of  old  age  is  repaired. 

475.  Spectacles. — The  most  common  and  simple  of  all  optical 
instruments  are  spectacles.     These  are  employed  to  remedy  the  defects 
of  eyes.     The  principles  involved  in  their  application  were  explained 
under  the  head  of  short-sightedness,  etc.  (459). 

Microscopes,  Opera-glasses,  Etc. 

476.  The  simple  microscope  is  a  simple  double  convex  lens, 
of  short  focal  distance.     These  are  called  magnifying  glasses,  and  are 
used  to  magnify,  to  an  ordinary  extent,  small  objects.     They  are  em- 
ployed by  various  artisans,  as   watch-makers,  engravers,  etc.,  whose 
labors  are  performed  on  minute  structures. 

Such  lenses  may  be  used  single  or  double.  They  are  usually  set  in 
a  rim  provided  with  a  handle,  or  fixed  in  a  short  cylinder  of  ivory  or 


OPTICS.  267 

horn.  Such  a  magnifying  glass  is  seen  at  L,  Fig.  12  (454).  These 
instruments  occupy  a  place  between  spectacles  and  regular  microscopes, 
composed  of  several  parts. 

Magnifying  power  of  a  lens  is  found,  for  ordinary  purposes, 
by  dividing  ten  inches  (the  limit  of  distinct  vision)  by  the  distance  of 
the  principal  focal  distance  of  the  lens. 

In  using  the  simple  microscope  the  object  is  placed  a  little  nearer  the 
lens  than  the  focal  distance,  in  which  case  the  divergent  rays  from  the 
object  will  be  made  to  pass  to  the  eye  as  parallel  rays,  and  the  object 
will  appear  as  large  as  if  the  eye  were  placed  at  the  optical  centre  of  the 
lens,  as  shown  by  Fig.  12  (454). 

If  the  focal  distance  be  half  an  inch,  the  magnifying  power  will  be 
10  -r-  $  =  20. 

^77.  Figure  16. — The  compound  microscope  consists  essen- 
tially of  a  double  convex  lens,  L,  called  the  object-glass,  and  a  second 
double  convex  lens,  F,  of  larger  size,  called  the  eye-piece. 

FIG.  16. 


The  object,  A,  is  placed  a  little  beyond  the  principal  focus  of  the 
object-glass  L.  This  lens  produces  a  real  image,  N,  which  is  inverted. 
The  eye-piece  or  lens,  F,  is  so  placed  that  its  principal  focus  is  a  little 
beyond  the  image  N.  This  then  acts  as  a  simple  microscope  and  mag- 
nifies the  image;  causing  it  to  appear  as  a  virtual  image,  in  the  situa- 
tion of,  and  as  large  as,  S. 

The  lenses,  of  course,  are  made  achromatic,  to  avoid  prismatic  colors 
(438). 

A  good  magnifying  power,  of  length  and  breadth,  is  600,  which,  be- 
ing squared,  gives  in  surface  360,000.  If  the  power  be  greater  than 
this,  distinctness  is  lost. 

The  object,  when  transparent,  is  illuminated  with  a  concave  mirror; 


208  OPTICS. 

when  opaque,  it  is  illuminated  by  concentrating  light  upon  it  with 
a  lens. 

The  microscope  is  employed  in  the  study  of  botany,  entomology, 
anatomy,  physiology,  and  for  many  purposes. 

To  find  the  power  of  a  compound  microscope,  multiply  the  power  of 
the  object-lens  by  the  power  of  the  eye-glass. 

478.  Figure  17. — The  magic  lantern. — This  is  an  apparatus 
for  projecting  upon  a  screen  enlarged  images  of  objects  painted  on  glass. 

FIG.  17. 


It  consists  of  a  dark  box,  in  which  a  lamp  is  placed  before  a  parabolic 
reflector;  the  light  being  concentrated  and  reflected  upon  a  plano-con- 
vex lens  L;  by  which  it  is  further  concentrated,  and  directed  upon  the 
object  painted  upon  the  glass  slide,  inserted  at  FF.  The  magnifying 
lens,  T,  is  placed  so  as  to  throw  its  focus  a  little  beyond  the  object, 
which  forms  an  image  of  the  illuminated  picture  upon  the  screen  S, 
placed  at  its  conjugate  focus. 

The  magnifying  power  is  equal  to  the  distance  of  the  screen  from 
the  lens  T,  divided  by  the  distance  of  the  lens  from  the  object.  There- 
fore the  power  of  the  instrument  depends  upon  the  lens  T. 

To  provide  for  the  adjustment  of  magnifying  lenses,  of  different 
powers,  to  the  painted  objects,  they  are  held  in  a  slide,  T. 

The  picture-object  should  be  inverted,  in  order  that  its  image  may 
appear  erect. 

479.  Figure  18. — The  solar  microscope  differs  in  princi- 
ple from  the  magic  lantern,  only  in  the  method  of  illuminating  the 
object. 

It  is  usually  employed  for  producing  on  a  screen  images  of  natural 
objects,  highly  magnified. 


OPTICS. 


269 


It  is  mounted  in  a  dark  room,  before  an  opening  in  a  shutter.  A 
plane  mirror,  M,  being  arranged  outside  the  shutter  in  such  a  position 
as  to  receive  the  direct  rays  of  the  sun,  and  reflect  them  through  the 
condensing  lens  H,  which  highly  illuminates  the  object  A,  the  object 
is  adjusted  to  the  focus  of  the  magnifying  lens  L,  and  a  greatly  en- 
larged image,  formed  at  the  conjugate  focus,  will  be  received  upon  a 
suitable  white  screen,  S. 

FIG.  18. 


The  magnifying  lens  may  be  a  small  globule  of  glass,  or  a  compound 
achromatic  object-glass  of  short  focus.  The  power  of  the  instrument 
depends  upon  this  lens. 

Instead  of  illuminating  the  object  with  light  of  the  sun,  electric 
light,  or  the  oxyhydrogen  light  may  be,  and  often  is,  employed,  and 
with  great  eifect.  For  some  purposes,  this  is  superior :  being  free  from 
the  heat  that  attends  a  concentration  of  the  solar  rays,  animalcules  are 
not  so  soon  destroyed,  and  the  instrument  may  be  used  in  any  place 
and  at  any  time. 

Upon  the  screen,  S,  is  represented  a  magnified  drop  of  vinegar;  the 
live,  snake-like  insects  being  sometimes  magnified  to  the  length  of  two 
or  three  feet,  which  are  seen  swimming  in  every  direction. 

Not  only  are  small  objects  brought  to  view,  but  minute  operations 
are  distinctly  exhibited,  as  the  manoeuvres  and  habits  of  insects,  the 
circulation  of  the  blood,  the  phenomena  of  crystallization,  and  a  great 
variety  of  Nature's  processes,  which,  to  the  unaided  eye,  would  forever 
remain  unobserved. 

The  objects  are  held  in  position  by  suitable  contrivances.  Live  in- 
sects, and  painted  ones,  and  drops  of  liquid  filled  with  animalculae,  dif- 
ferent kinds  of  vegetable  substances,  animal  tissues,  etc.,  constitute 
interesting  objects. 


270  OPTICS. 

480.  Polyrama  and  dissolving  views.— The  polyrama  con- 
sists of  a  double  magic  lantern.     The  dissolving  views  are  obtained  by 
using  both  lanterns. 

If  a  scene,  painted  by  moonlight,  be  put  upon  one  side  in  the  lantern, 
and  a  painting  of  the  same  scene  by  daylight  be  put  upon  the  other 
side,  of  course,  first  one  scene  and  then  the  other  can  be  thrown  upon 
the  screen,  by  alternately  covering  and  uncovering  the  two  tubes  of  the 
lanterns.  But  by  gradually  cutting  off  the  light  from  one  picture,  and, 
at  the  same  time,  gradually  admitting  it  to  the  other,  the  first  will 
insensibly  fade  away,  whilst  the  other  as  insensibly  grows  brighter.  In 
this  manner  all  the  effects,  intermediate  between  full  daylight  and  full 
moonlight,  may  be  obtained  in  succession. 

481.  Figure  19. — Opera-glasses. — The  opera-glass  consists  es- 
sentially of  a  convex  object-lens,  which  collects  the  rays,  and  a  concave 
lens  as  an  eye-piece,  by  means  of  which  the  rays  from  each  point  of  the 
object  are  rendered  parallel,  and  thus  capable  of  producing  distinct 
vision. 

FIG.  19. 


The  object-glass,  N,  converges  the  rays  coming  from  the  object  A, 
upon  the  concave  eye-glass,  L,  by  which  the  converging  rays  are  ren- 
dered slightly  divergent  before  entering  the  eye,  as  though  emanating 
from  the  position  T,  at  a  distance  of  distinct  vision ;  the  image  being 
virtual  and  erect. 

The  large  object-glass,  of  long  focal  distance,  and  the  eye-piece,  of 
short  focal  distance,  are  placed  at  a  distance  apart  equal  to  the  differ- 
ence of  their  principal  foci,  which  collects  a  large  number  of  rays  from 
the  object  and  brings  them  to  such  a  state  of  divergence  as  to  produce 
distinct  vision  in  the  eye. 

As  the  eye-piece  is  concave,  the  magnifying  power  of  such  an  instru- 


OPTICS.  271 

ment  is  found  by  dividing  the  principal  focal  distance  of  the  convex 
lens  by  the  principal  focal  distance  of  the  concave  leris. 

This  instrument  was  first  constructed  by  Galileo  and  employed  as  a 
telescope  ;  hence  it  is  called  the  Galilean  telescope. 

When  employed  as  an  opera-glass  it  is  made  double  to  provide  for 
both  eyes. 


Night-glasses,  employed  by  seamen,  have  the  same  con- 
struction as  opera-glasses,  except  they  are  larger  and  have  less  magni- 
fying power;  the  object  being  to  concentrate  a  large  amount  of  light 
in  such  a  condition  as  to  allow  of  distinct  vision,  to  enable  the  observer 
to  see  objects  distinctly  at  night. 

The  Camera  Obscura. 

483.  Figure  20.  —  The  camera  obscura,  as  employed  by 
artists  for  tracing  landscapes,  etc. 

Having  explained  the  general  principles  of  the  camera  obscura  in 
connection  with  the  description  of  the  eye  (439),  it  only  remains  to 
show  the  manner  in  which  it  is  adapted  to  the  use  of  the  artist. 

FIG.  20. 


In  the  dark  box  is  provided  a  plane  mirror,  M,  inclined  at  an  angle 
of  45°,  upon  which  the  rays  S,  coming  from  the  object  or  scenery,  are 
received,  and  by  which  they  are  reflected  upward  to  a  glass  plate  or 
transparent  screen,  upon  which  is  laid  the  paper  P,  and  on  which  the 
tracing  is  made.  The  lens,  in  L,  gives  the  proper  convergence  to  the 
rays.  The  box  is  placed  on  a  stand  or  any  convenient  support.  The 
cover,  T,  serves  to  protect  the  glass  screen  when  the  instrument  is  not 
in  use. 


OPTICS. 


484-  Figure  21. — Another  form  of  the  camera  obscura. 

Upon  the  frame,  FF,  is  mounted  a  horizontal  head-piece,  in  which  is 
FIG.  21.  fitted  a  plane  mirror,  M.     To  exclude 

the  light,  a  black  cloth  is  thrown  over 
the  frame.  The  rays  of  the  object,  A,  or 
scenery,  are  received  upon  the  mirror 
and  reflected  through  the  lens,  and  re- 
ceived upon  the  paper  laid  on  the  table, 
H,  below,  as  shown.  The  lens  is  held 
in  a  sliding  tube,  TT,  to  render  its  focus 
adjustable.  Different  lenses  are  em- 
ployed. The  artist  seats  himself  under 
the  cloth  (not  shown)  thrown  over  the 
frame-work,  FF. 

485.  The  camera  lucida  is  an- 
other instrument  employed  for  sketch- 
ing from  nature.  It  consists  of  a  prism 
having  one  right  angle  and  two  angles 
of  135°,  by  which  total  reflection  (399) 
takes  place ;  or  the  prism  may  have  one 
^^^^^^^^™  right  angle,  and,  opposite  to  this,  an 
angle  of  135°,  and  two  other  angles  of  67|°  each.  In  this  case  the 
light  will  be  twice  totally  reflected,  entering  the  eye  in  the  direction 
in  which  the  object  will  be  seen.  Hence,  the  instrument  may  be  so 
placed  that  the  object  will  be  seen  on  the  paper  where  it  is  to  be 
traced.  The  dimensions  of  the  image  will  be  as  much  smaller  than 
those  of  the  object,  as  the  distance  of  the  prism  from  the  paper  is  less 
than  its  distance  from  the  object ;  therefore,  by  varying  the  position 
of  the  prism,  the  size  of  the  image  can  be  varied  to  suit  the  occasion. 

486.  Daguerreotyping  is  the  art  of  producing  pictures  by  the 
actinic  or  chemical  action  of  light;  involving,  besides  the  chemical 
action,  all  the  principles  of  the  camera  obscura. 

Instead  of  receiving  the  image  on  a  screen  of  paper,  to  be  traced  with 
pen  or  pencil,  it  falls  on  a  metallic  or  glass  plate,  previously  made  sen- 
sitive to  the  action  of  light,  by  iodine,  bromine,  or  other  chemical  prep- 
aration. The  action  of  the  light  upon  the  chemicals  is  such,  that,  by 
certain  chemical  treatment  of  the  plate  by  the  artist  in  the  dark  labora- 
tory, the  image  is  further  developed  and  fixed. 

The  daguerreotype,  ambrotype,  crystallotype,  etc.,  are  thus  produced. 
Of  course,  there  are  minor  details  connected  with  the  art,  which  it  is 
not  necessary  to  describe. 

The  achromatic  compound  lens  is  employed  in  the  camera. 


OPTICS.  273 

Jf87 *  Photography  is  the  art  of  fixing  upon  paper  the  picture 
produced  by  the  camera.  If  the  picture  be  taken  by  the  camera  upon 
glass,  it  has  the  lights  and  shades  reversed,  and  is  called  a  negative.  By 
laying  the  negative  upon  chemically  prepared  paper,  the  action  of  the 
sunlight  reverses  the  position  of  the  lights  and  shadows  of  the  picture 
on  the  paper,  which,  being  fixed  by  further  chemical  treatment,  may  be 
pasted  on  card-board  for  use. 

Any  number  of  copies  may  be  made  or  printed  from  the  same  nega- 
tive. 

Telescopes. 

Jj,88.  The  different  kinds  of  telescopes. — A  telescope  is  an 
optical  instrument  for  viewing  objects  at  more  than  ordinary  distances ; 
and,  in  effect,  to  bring  them  apparently  nearer  to  the  eye,  by  increasing 
the  apparent  angles  under  which  such  objects  are  seen. 

Telescopes  are  first  divided  into  two  classes,  refracting  telescopes  and 
reflecting  telescopes. 

In  the  first  class,  an  object-lens  is  used  to  form  an  image;  in  the 
second  class,  a  speculum  or  mirror  is  employed  for  this  purpose.  In 
both  classes  the  image  thus  formed  is  viewed  by  a  lens,  or  combination 
of  lenses,  termed  the  eye-piece. 

The  manner  in  which  the  component  parts  are  arranged,  together 
with  the  nature  of  the  auxiliary  pieces,  determines  the  particular  kind 
of  telescope. 

Jf89.  Figure  22.  —  The  refracting  astronomical  tele- 
scope.— This  instrument  consists  essentially  of  two  convex  lenses, 
the  one,  L,  being  the  object-glass,  and  the  other,  IN",  the  eye-piece. 

The  pencils  of  rays  coming  from  the  object,  A,  are  converged  by  L 
to  a  focus,  forming  the  real  inverted  image,  T.  The  eye-piece,  N,  is 

FIG.  22. 


placed  at  a  distance  from  the  image,  T,  equal  to  its  principal  focal  dis- 
tance. The  pencils  of  light  from  this  image  are  refracted  and  con- 
verged to  the  eye,  so  as  to  form  a  visual  angle  many  times  larger  than 
it  would  be  if  the  object  were  viewed  with  the  naked  eye ;  and,  conse- 
quently, the  object  appears  to  be  magnified. 

18 


274 


FIG.  23. 


OPTICS. 

All  telescopes  are  rendered  adjustable,  to 
view  objects  at  different  distances,  by  alter- 
ing the  position  of  the  eye-piece,  which,  for 
this  purpose,  is  set  in  a  sliding  tube. 

The  object-lens  should  be  achromatic,  to 
prevent  prismatic  colors ;  but  slightly  con- 
vex, to  increase  the  distance  between  the 
lenses ;  and  as  large  as  possible,  to  illumi- 
nate the  image. 

The  magnifying  power  is  found  by  divid- 
ing the  principal  focal  length  of  the  object- 
glass  by  that  of  the  eye-glass. 

Of  course,  the  image,  and,  therefore,  the 
object,  will  be  seen  inverted,  but  this  is  not 
objectionable  in  viewing  heavenly  bodies. 

One  of  the  finest  telescopes  of  this  class 
in  the  world,  is  in  the  Observatory  at  Chi- 
cago, Illinois.  Its  object-glass  is  18  inches 
diameter.  This  instrument  takes  in  about 
6,000  times  as  much  light  as  the  eye. 

490.  Figure  23.— The  terrestrial 
telescope  or  spy-glass. — The  princi- 
ples involved  in  the  construction  of  this 
instrument  are  the  same  as  in  the  one  just 
described;  but  as  it  is  desirable  to  see  ter- 
restrial objects  erect,  it  becomes  necessary 
to  invert  the  inverted  image.  This  is  ac- 
complished by  the  introduction  of  two  other 
lenses,  besides  the  usual  object-glass  and 
eye-piece. 

The  diagram  shows  the  course  of  the  rays 
in  a  terrestrial  telescope.  The  arrow  in 
front,  or  object,  is  supposed  to  be  remote 
from  the  instrument. 

The  object-lens,  1,  forms  the  inverted 
image,  L ;  the  lens  2  converges  the  rays  of 
the  image  to  a  focus  between  the  inverting 
lenses  2  and  3,  where,  after  crossing,  they 
pass  on  and  diverge  to  the  second  inverting 
lens,  3,  and  are  brought  to  a  focus  between 
the  lenses  3  and  4,  forming  the  erect  image, 
T ;  which  is  magnified  by  the  eye-piece,  4. 


OPTICS. 


275 


The  several  tubes,  which  slide  one  within  another,  allow  the  instru- 
ment to  be  reduced  to  a  convenient  length  when  not  in  use. 

491.  Figure  24.— HerschePs  reflecting  telescope.— There  is 
a  great  variety  of  reflecting  telescopes,  in  all  of  which  a  parabolic  metallic 
speculum  or  mirror  (392)  is  employed,  instead  of  the  object-lens,  to  form 
an  image  of  the  object,  and  an  eye-piece  is  used  to  magnify  the  image. 

The  figure  represents  Sir  William  Herschel's  telescope.  A  A  is  a 
sheet-iron  tube,  in  one  end  of  which  is  a  parabolic  speculum,  M,  some- 

FIG.  24. 


what  less  in  diameter  than  the  tube,  with  its  axis  directed  to  one  side 
of  the  tube,  shown  by  the  dotted  line.  The  parallel  rays,  EF,  from 
some  very  distant  object,  are  received  upon  the  mirror  and  reflected, 
converging,  to  the  eye-piece,  L.  The  size  of  the  tube  and  the  inclina- 
tion of  the  axis  of  the  speculum  are  so  adjusted,  that  the  observer  does 
not  intercept  any  light  which  can  fall  upon  the  reflector. 

This  is  called  the  front-view  telescope.  The  speculum  of  Herschel's 
great  telescope  was  4  feet  in  diameter,  3£  inches  thick,  weighing  2,118 
pounds,  with  a  focal  distance  of  40  feet,  and  magnifying  power  of  6,450 
diameters. 

This  telescope  is  a  modification  of  the  Newtonian  telescope. 

FIG.  25. 


492.  Figure  25. — The  Gregorian  reflecting  telescope.— 

M  is  a  concave  metallic  speculum,  having  a  hole  in  its  centre.     This 


276 


OPTICS. 


JTIG 


reflector  will  form,  at  its  focal  distance,  an  inverted  image,  L.  At  T  is 
placed  a  small  concave  mirror,  about  one-fourth  the  focus  and  diameter 
of  the  speculum,  M,  and  facing  toward  it,  and  at  a  little  greater  distance 
from  L  than  its  own  focal  distance.  Kays  diverging  from  L  are  rendered 
less  divergent  after  reflection  by  T,  and  are  thrown  back,  in  nearly  paral- 
lel lines,  to  the  plano-convex  eye-piece,  F,  by  which  they  are  brought 
to  a  focus,  forming  an  erect  image.  The  rays  then  passing  the  second 
eye-piece,  E,  are  converged  at  the  eye,  where  the  object  seems  to  appear 
under  a  much  enlarged  visual  angle,  shown  by  the  two  dotted  lines. 

493.  Figure  26.—  The  Newtonian  reflecting  telescope, 

as  improved  and  constructed 
by  M.  Froment,  of  Paris. 

M  is  a  concave  reflector, 
placed  at  the  bottom  of  a 
long  tube.  The  reflector 
tends  to  form  a  small  image 
of  the  object,  A,  at  the  other 
end  of  the  tube  ;  but  before 
the  rays  reach  the  image 
they  are  intercepted  by  the 
glass  prism,  L,  so  arranged 
that  the  rays,  entering  its 
first  face,  will  be  totally  re- 
flected, and  form  an  im- 
age of  the  object  at  F.  This 
image  is  viewed  by  an  eye- 
piece through  the  side  of  the 
telescope,  as  represented. 
The  eye-piece  is  made  of 
two  plano-convex  lenses,  the 
combined  effect  of  which  is 
to  cause  the  image  to  appear 
under  the  much  enlarged 
visual  angle  indicated  by  the 
two  dotted  lines  H  and  N. 


494-  kor(l  Rosse's 
reflecting  telescope  lias 
a  tube  56  feet  long  by  7  feet 
diameter,  with  a  speculum  of 
6  feet  diameter,  weighing  4 

tons  ;  and  the  entire  instrument  weighs  more  than  18  tons,  and  cost 

$60,000. 


OPTICS.  277 

495.  Figure  27. — The  telestereoscope. — Owing  to  the  fact 
that  the  two  eyes  do  not  view  an  object  from  the  same  point  (447),  the 
image  formed  on  the  two  retinae  are  not  exactly  alike  (469),  and  by  the 
difference  in  the  images  we  are  aided  in  judging  of  the  distance  and 
figure  of  the  object  (466).  The  nearer  the  object  the  greater  is 
this  difference.  If  the  object  is  very  distant  the  images  will  be  sensibly 
identical,  and  we  lose  the  aid  just  mentioned,  in  estimating  the  dis- 
tance and  bodily  figure. 

The  object  of  the  telestereoscope  is  to  increase  the  optic  angle  or  binocu- 
lar parallax  (447)  of  distant  objects,  by  presenting  to  each  eye  such  a  view 

FIG.  27. 


as  would  be  obtained  if  the  distance  between  the  eyes  were  greatly  in- 
creased, which  increases  the  difference  in  the  images  on  the  two  retinae, 
and  gives  the  same  appearance  of  relief  to  the  object  as  if  it  were 
brought  near  to  the  observer. 

Let  AB,  rays  of  light  coming  from  some  distant  object,  fall  upon  the 
two  mirrors,, MM,  and  be  reflected  to  the  two  mirrors,  T,  and,  being 
again  reflected  from  these  mirrors  to  the  eyes,  OT",  of  the  observer,  the 
two  views  seen  will  evidently  be  the  same  as  if  the  eyes  were  separated 
to  the  positions  of  J  and  K. 

The  relief  with  which  objects  are  seen  by  this  instrument  is  increased 
as  much  as  the  distance  between  J  and  K  exceeds  that  between  the 
eyes,  NN. 

Though  the.  perspective  difference  of  the  images  seen  by  the  two 
eyes  is  increased,  the  visual  angle  under  which  each  object  is  seen  re- 
mains unchanged,  and  hence,  as  the  apparent  distance  of  the  object  is 
diminished,  their  dimensions  appear  diminished  in  the  same  pro- 
portion. 


278  OPTICS. 

If  lenses  (such  as  are  used  in  opera-glasses)  are  inserted,  the  object- 
glasses  being  placed  at  FF,  between  the  large  and  small  mirrors,  and 
the  concave  eye-pieces  between  the  small  mirrors  (T)  and  the  eyes,  the 
effect  will  be  to  increase  the  visual  angle  of  every  object  in  the  field 
of  view. 

If  the  glasses  magnify  as  many  diameters  as  the  distance  between  J 
and  K  exceed  the  distance  between  the  eyes,  every  object  will  appear  in 
its  due  proportions,  and  the  appearance  will  be  as  though  the  observer 
had  been  transported  to  the  immediate  vicinity  of  the  objects  themselves. 

The  distance  between  the  large  mirrors  should  not  exceed  the  breadth 
of  an  ordinary  window,  unless  the  instrument  is  to  be  used  in  the 
open  air. 

Jj,96.  Figure  28. — The  stereoscope  (from  words  signifying 
solid  and  to  see)  is  an  instrument  by  which  two  flat  pictures  are  made 
to  appear  like  a  single  solid  or  projecting  body. 

FIG.  28. 


This  figure  represents  the  exterior  appearance  of  the  instrument. 
The  pictures  are  inserted  at  the  opening  seen  on  the  right  ;  the  lid  at 
the  top  admits  and  regulates  the  light  on  the  pictures.  The  pictures 
are  seen  through  the  tubes. 


Figure  29.  —  The  principles  of  the  stereoscope.— 

This  instrument  is  constructed  upon  the  principles  explained  in  con- 
nection with  Fig.  27  (495).  The  two  pictures  to  be  viewed  by  the 
stereoscope  are  not  taken  from  the  same  point  of  view.  In  taking 
stereoscopic  photographs  of  near  objects,  one  picture  is  taken  by  plac- 
ing the  camera  in  the  position  of  the  left  eye,  and  the  other  by  placing 
it  in  the  position  of  the  right  eye.  If  the  scene  or  object  to  be  photo- 
graphed be  distant,  the  two  points  from  which  they  are  taken  must 
be  wide  apart.  Pictures  thus  taken,  when  viewed  by  the  stereoscope, 


OPTICS. 


279 


will  stand  out  in  relief  as  the  scene  or  object  itself  would  if  viewed  by 
direct  vision. 

Let  a  corresponding  point  of  each  picture  be  represented  by  FF,  and 
rays  from  these  points,  falling 
upon  the  semi-double  convex 
lenses,  HH,  will  be  refracted  to 
the  eyes,  EE,  and  the  mind  will 
refer  both  points,  FF,  to  the 
central  position  L.  What  is 
true  of  these  two  points,  FF,  is 
true  of  all  other  points  of  the 
two  pictures. 

As  one  picture  represents  the 
real  or  projecting  object,  as  .ob- 
served by  the  right  eye,  and  the 
other  as  seen  by  the  left  eye 
(though  appearing,  when  viewed 
through  the  lenses  HH,  to  pro- 
ceed from  the  same  object),  the 
impression  made  on  the  mind 
will  be  the  same  as  if  both  images  were  derived  from  one  solid  or  pro- 
jecting body,  instead  of  from  two  somewhat  unlike  flat  pictures. 

The  distance  between  the  two  positions  in  which  the  camera  is 
placed  to  take  stereoscopic  pictures,  varies  from  a  few  inches  to  any 
distance  necessary  to  produce  the  desired  effect. 

498.  Figures  30  and  31.— The  stereomonoscope.— This 

is  an  instrument  by  which  a  single  image  is  made  to  present  the  ap- 

FIG.  30. 


pearance  of  relief,  as  seen  in  the  stereoscope,  and  by  means  of  which 
several  persons  can  view  these  effects  at  the  same  time. 


280  OPTICS. 

If  an  object,  T,  be  placed  before  a  large  double  convex  lens,  L,  an 
image  of  the  object  will  be  formed  at  the  conjugate  focus,  which  may 
be  received  on  a  plate  of  ground  glass,  P.  From  the  image,  rays  of 
light  will  diverge,  as  from  a  real  object,  which  will  be  seen  wherever 
the  eyes  may  be  placed,  within  the  cone  of  diverging  rays,  as  shown  by 
the  figured  eyes  on  the  right. 

The  stereomonoscope  (Fig.  31)  consists  of  two  such  lenses,  L  and  N, 
so  placed  as  to  form  images  of  stereoscopic  pictures,  A  and  B,  on  a 

FIG.  31. 


screen  of  ground  glass,  P.  Though  the  two  pictures  have  their  images 
superimposed  on  the  same  part  of  the  screen,  P,  yet  each  picture  can 
be  seen  only  by  the  rays  emanating  from  the  photograph  by  which  it 
was  formed. 

If  the  eyes  be  placed,  as  figured,  so  that  rays  coming  from  one  lens 
will  enter  the  right  eye,  and  those  from  the  other  lens  the  left  eye,  the 
object  (from  which  the  pictures  were  taken)  will  appear  in  relief,  as  in 
the  stereoscope.  Several  persons  can  witness  the  effect  at  the  same 
time. 

WAVE    THEORY    OF    LIGHT. 

Interference,  Diffraction,  etc. 

^,99.  Figure  32. — Waves  of  light. — As  previously  stated 
(356),  the  undulatory  or  wave  theory  is  most  generally  received.  Ac- 
cording to  this  theory,  the  cause  of  light  is  an  undulatory  movement 
in  the  ethereal  medium.  In  this  elastic  medium  undulatory  movements 
can  be  propagated  in  the  same  manner  as  waves  of  sound  in  air.  The 
ether  and  light  are  not  the  same.  The  latter  is  the  effect  of  movements 
in  the  former;  as  air  is  one  thing,  and  the  sound  which  traverses  it 
another.  These  waves  advance  at  the  rate  of  192,000  miles  per  second. 
The  particles  of  ether  do  not  advance  at  this  rate,  but  only  the  waves. 
This  may  be  illustrated  thus: 


OPTICS. 


281 


Having  fastened  one  end  of  a  cord  to  a  fixed  obstacle,  F,  commence 
agitating  the  end  A, 

,      ,  1,1  -T  Iw« 

up  and  down,  and  the 

cord   will    be   thrown 

into  wave-like  motions, 

passing    rapidly   from 

one  end  to  the  other. 

The  particles  composing  the  cord  do  not  advance  or  retreat,  however 

rapidly  the  undulations  may  pass.     So,  too,  floating  objects  on  water 

only  rise  and  fall  with  waves,  when  the  waves  pass  on  ;  thus  showing 

that  the  water  itself  does  not  advance  forward  with  its  undulations. 

The  vibration  is  the  cause  of  undulation.  In  case  of  the  cord,  the 
vibration  is  represented  by  the  movement  exerted  by  the  hand;  the 
undulation  is  the  wave-like  motion. 

As  a  vibrating  string  agitates  the  surrounding  air,  and  makes  waves 
of  sound  pass  through  it,  so  does  an  incandescent  or  shining  particle, 
vibrating  with  surprising  rapidity,  impress  a  wave-like  movement  on 
the  ether,  and  this  movement,  finally  impinging  on  the  eye,  is  what  we 
term  sight. 

500.  Figure  33. — Directions  of  vibrations  and  waves  of 
light. — If  the  free  end  of  the  cord  be  vibrated  horizontally,  vertically, 
or  diagonally  up  and  down,  as  indicated  by  the  arrow-heads,  or  in  any 
intermediate  directions,  these  directions  will  all  be  transverse,  or  at 


FIG.  33. 


right  angles  to  the  length  of  the  cord,  or  the  direction  of  the  waves. 
This  is  the  peculiarity  of  the  movement  of  light ;  that  is,  its  vibrations 
are  transverse  to  the  course  of  the  ray.  With  sound,  the  vibrations  are 
executed  in  the  direction  of  the  resulting  wave,  and  not  at  right  angles 
to  it.  Hence,  the  undulatory  theory  of  light,  by  some  writers,  is  desig- 
nated the  Theory  of  Transverse  Vibrations. 


2&%  OPTICS. 

501.  Brilliancy  dependent  on  amplitude  of  waves.— 

Lights  differ  from  each  other  in  brilliancy  and  color,  which  depend  on 
qualities  in  the  waves.  As  waves  of  water  may  vary  in  height,  by 
which  is  meant  amplitude,  so  waves  of  light  vary  in  amplitude.  A 
wave  of  great  amplitude  impresses  us  with  a  sense  of  intensity  or  bril- 
liancy, while  a  wave  of  small  amplitude  is  less  brilliant.  Therefore, 
the  brilliancy  of  light  depends  on  the  magnitude  of  the  excursions  of 
the  vibrating  particles,  as  the  amplitude  of  the  waves  in  the  cord 
(Fig.  33)  depends  upon  the  distance  which  the  hand  is  moved,  which 
causes  the  waves. 

502.  Color  dependent  on  length  of  waves.— By  length 
of  wave  is  meant  the  distance  from  the  crest  of  one  wave  to  that  of  the 
next,  as  from  A  to  T,  Fig.  32  (499),  or  from  depression  to  depression. 
The  length  of  the  waves  determines  the  color  of  light.     The  longer 
waves  give  rise  to  red  light;  the  shorter  ones,  to  violet,  and  those  of 
intermediate  lengths,  the  other  colors,  in  the  order  of  their  refrangibility. 

503 .  Figure  34. — Interference  of  light.— If  two  waves  of 
water  encounter  in  such  a  manner  that  the  concavity  of  the  one  corre- 
sponds with  the  convexity  of  the  other,  they  mutually  destroy  each 
other's  effect.     So  it  is  with  waves  of  sound.     If  waves  thus  encounter 
they  destroy  each  other's  effect.     Hence,  two  sounds,  at  the  point  of 
their  encounter,  produce  silence.     In  like  manner,  if  two  waves  of  light 
similarly  encounter  they  destroy  each  other's  effect.     Therefore,  two 
rays  of  light,  however  brilliant  they  may  be  separately,  will  produce 
darkness  at  these  points  of  encounter.     This  is  called  interference  of 
light. 

FIG.  34. 


Let  A  and  B  represent  two  encountering  rays  of  light,  in  which  the 
two  systems  of  waves  or  undulations  are  in  opposite  phases,  the  con- 
vexity of  the  one  corresponding  with  the  concavity  of  the  other, 
and  interference  will  take  place,  as  at  L,  Droducing  darkness  at  this 
point. 

504.  Figure  35.— Non-interference  of  light.— If  two  rays 
of  light,  as  A  and  B,  encounter  each  other  in  such  a  manner  that  the 


OPTICS. 


283 


concavities  and  convexities  of  their  undulations  respectively  corre- 
spond, there  is  no  interference ;  and  where  they  encounter,  as  at  L, 

FIG.  35. 


instead  of  darkness  being  the  result,  an  intenser  light  is  produced  at 
this  point. 

505.  Figure  36.— Demonstration  of  interference  of  light. 

— Produce  a  lucid  point  at  S,  by  bringing  rays  of  the  sun  to  a  focus  by  a 
double   convex   lens,  or  by  passing  a  FIG.  36. 

sunbeam  through  a  pin-hole.  In  the 
diverging  rays  from  this  lucid  point 
place  a  cylindrical  body,  as  a  piece  of 
wire,  F  (seen  endwise  in  the  figure); 
at  some  distance  beyond,  place  a  screen 
of  white  paper.  The  object,  F,  will 
throw  a  shadow  on  the  screen,  reach- 
ing from  H  to  "W.  This  shadow,  in- 
stead of  being  uniformly  dark,  is  found 
to  consist  of  light  and  dark  stripes,  as 
represented  by  the  figure  A,  caused  by 
interference.  The  cause  of  the  inter- 
ference is  thus  explained : 

Waves  of  water  pass  round  to  the 
back  of  an  object  on  which  they  im- 
pinge, and  the  undulations  of  light,  in 
the  same  manner,  flow  round  at  the 
back  of  the  piece  of  wire,  F.  The  two 
series  of  waves  which  have  passed  from 
the  opposite  sides  of  the  obstacle  to  the 
middle  point,  N,  of  its  shadow,  having 
passed  through  paths  of  equal  length, 
will  encounter  in  such  a  manner  as 
not  to  interfere;  and,  therefore,  they 
will  exalt  each  other's  effect,  and  pro- 
duce a  light  line  at  this  point. 

The   systems  of  waves   which   have 
passed   from  the   sides  of  the  obstacle  to  the  point,  Y,  having  come 
through  distances  which  differ  in  length  by  half  a,  wave,  will  encounter 


284  OPTICS. 

at  Y  in  such  a  manner  as  to  interfere  and  destroy  each  other's  effect, 
and  so  produce  a  dark  stripe  at  this  point. 

At  the  point,  L,  the  waves  from  each  side  of  the  obstacle  again  have 
come  through  unequal  paths  which  differ  in  length  by  a  whole  wave, 
and,  therefore,  they  will  again  encounter  in  such  a  manner  as  not  to 
interfere,  and  another  white  stripe  is  produced. 

The  correctness  of  this  explanation  is  shown  by  placing  an  opaque 
screen  on  one  side  of  the  obstacle,  F,  so  as  to  prevent  the  light  passing, 
when  the  fringes  will  disappear. 

506.  Laws  of  interference    and   non-interference    of 
light. — 1.  If  t'wo  systems  of  waves,  of  the  same  length,  encounter  after 
having  come  through  paths  of  equal  length,  they  will  not  interfere. 

2.  Nor  will  they  interfere  though  there  be  a  difference  in  the  length  of 
these  paths,  provided  that  difference  be  equal  to  one  whole  wave,  or  two, 
or  three,  etc. 

3.  But  if  the  paths  be  of  unequal  length,  they  will  interfere,  and 
the  interference  will  be  complete  when  the  difference  of  the  length  of  the 
paths  is  half  a  wave,  1  £,  2£,  3  J,  etc. 

507 .  Figure  37.— Interference  colors  are  seen  in  thin  films 
of  varnish,  cracks  in  glass,  and  other  thin  transparent  substances,  as 
in  soap-bubbles. 

Let  FH  represent  a  section  of  a  thin  transparent  bulb  of  glass  or  of 
a  soap-bubble.  If  a  ray  of  light,  S,  is  incident  at  V,  a  portion  of  the 
light,  after  refraction  and  transmission,  will  pass  on  in  the  direction  of 


FIG.  3 


K,  and  another  portion  will  be  reflected  at  V,  as  shown  by  the  first 
vertical  arrow.  At  the  point,  A,  a  portion  of  the  light  is  internally 
reflected  from  the  second  surface,  and  is  divided  by  transmission  and  a 
second  internal  reflection  at  the  first  surface,  the  reflected  portion  being 
transmitted  in  the  direction  of  J. 


OPTICS.  285 

The  curves,  in  the  rays  J  and  K,  represent  waves.  The  second  ray 
at  the  first  surface,  reflected  from  the  second  surface,  will  be  retarded 
behind  the  first  a  distance  equal  to  twice  the  thickness  of  the  bubble, 
shown  by  the  difference  in  the  length  of  the  two  vertical  arrows.  The 
ray  J,  having  traversed  the  thickness  of  the  film  twice,  will  fall  behind 
the  ray  K  a  distance  equal  to  twice  the  thickness  of  the  bubble.  If 
these  retardations  equal  the  interval  of  an  odd  number  of  half  waves 
they  will  interfere,  and  produce  dark  lines. 

The  difference  in  the  intensity  of  the  rays  J  and  K  being  greater 
than  that  of  the  two  rays  proceeding  from  the  first  surface,  the  dark 
lines  in  the  former  are  less  distinct  than  in  the  latter. 

The  difference  in  the  dark  and  bright  bands  thus  produced  is  differ- 
ent for  different  colors  of  the  spectrum,  being  least  for  violet  and 
greatest  for  red.  The  dark  bands  and  peculiar  tints  of  the  soap-bub- 
ble are  thus  due  to  interference. 

508.  Figures  38  and  39.— Determining  the  length  of 
waves  of  light. — The  thickness  of  soap-bubbles  cannot  be  accu- 
rately measured ;  but  dark  rings  can  be  produced  by  other  means, 
which  facilitate  the  measurement  of  the  distances  between  the  reflect- 
ing surfaces. 

Place  upon  a  flat,  smooth  plate  of  glass  another  slightly  curved  piece, 
whose  curvature  is  that  of  a  por-  FlG  ^ 

tion  of  a  sphere  whose  radius  is  40 
or  50  feet,  as  represented  by  Fig.  38. 

When  this  curved  glass  is  pressed 
down  upon  the  plate,  the  centre 
appears  black,  and  is  surrounded 
by  colored  rings  (Fig.  39),  as  in  the  soap-bubble.  If  homogeneous 
light,  as  red,  be  allowed  to  fall  vertically  upon  the  upper  glass,  rings 
will  be  formed  at  1,  2,  3,  and  4;  and  the  diameter  of  these  rings, 
shown  by  the  dotted  lines,  can  be  easily  measured.  They  are  always 
found  to  be  in  the  proportion  of  1  —  1.414  -1.723  —  2.000,  and  so  on. 
These  numbers  are  the  square  roots  of  1,  2,  3,  .4,  and  so  on ;  and  it  is 
known,  from  the  form  of  the  sphere,  that  the  distances  IE,  2N,  3S, 
4H,  etc..  are  to  one  another  as  the  squares  of  the  cords  or  dotted 
diameters.  Hence  the  distance  between  the  reflecting  surfaces  of  the 
second  bright  ring  is  twice  that  of  the  first,  and  so  on.  The  diameters 
of  the  bright  rings  being  as  the  square  roots  of  1,  2,  3,  4,  and  so  on, 
the  diameters  of  the  dark  rings  will  be  as  the  square  roots  of  1£,  2J, 
3J,  4J,  etc. 

The  distance  between  successive  rings  of  violet  will  be  much  less 
than  that  between  successive  rings  of  red. 


286  OPTICS. 

509.  Length  of  waves  or  undulations  of  light  (Fig.  39). 
— The  light  reflected  from  E  must  travel  half  a  wave-length  (see  3d 
law  of  interference,  506)  further  than  that  reflected  from  1,  in  order 
that  the  waves  reflected  from  these  points  may  meet  in  the  same  phase, 
and  so  give  a  bright  ring.  But  the  wave  reflected  from  K  travels  over 
the  space  1R  twice;  therefore  1R  must  be  only  ^  the  length  of  a 
luminous  wave.  But  1R,  as  previously  shown,  is  £  of  4H.  The  length 
of  4H  is  easily  found.  Thus :  4F  is  half  the  diameter  of  the  fourth 

FIG.  39. 


ring,  and  can  be  found  by  actual  measurement;  and  the  length  of  the 
radius,  4E,  is  known,  and  4FE  is  a  right  angled  triangle.  The  hypoth- 
enuse,  4E,  and  the  side,  4F,  being  known,  the  length  of  EF  is  found. 
Having  found  EF,  and  knowing  EK,  4H  is  found  by  subtracting  EF 
from  EK,  or  EK  —  EF  =  4H. 

By  this  and  other  methods,  the  length  of  waves,  or  undulations,  re- 
quired to  produce  different  colors,  has  been  estimated,  and  the  num- 
ber of  waves  that  enter  the  eye  per  second. 

The  length  of  the  vibrations  in  the  extreme  red  ray  is  just  double 
the  length  of  the  vibrations  of  the  invisible  rays  beyond  the  violet, 
which,  concentrated,  produce  the  lavender  light  of  Herschel.  The 
entire  range  of  rays,  therefore,  extends  only  over  what  is  equivalent  to 
a  single  octave  of  music. 

The  following  table  exhibits  the  numerical  results  which  have  been 
deduced  for  the  length  and  velocity  of  luminous  waves  of  different 
colors. 


OPTICS. 


287 


COLORS. 

LENGTH  OP  UNDULA- 
TIONS  IN  PARTS  OP 
AN  INCH. 

NUMBER  OP  UNDULA- 
TIONS IN  AN  INCH. 

NUMBER  OP  UNDULATIONS 
PER  SECOND. 

Extreme  red  
Red 

0.0000266 
0  0000256 

37640 
39180 

458,000000,000000 
477  000000  000000 

Orange  

0.0000240 

41610 

506  000000  000000 

Yellow  

0.0000227 

44000 

535  000000  000000 

Green         

0  0000211 

*47460 

577  000000  000000 

Blue  

0.0000196 

51110 

622  000000  000000 

Indigo            .  . 

0  0000185 

54070 

658  000000  000000 

Violet       

0.0000174 

57490 

699  000000  000000 

Extreme  violet  .  . 

0.0000167 

59750 

727,000000,000000 

510.  The  cause  of  the  waves  of  light  is  supposed  to  be  the 
vibrations  of  the  particles  of  a  luminous  body. 

In  ordinary  combustion,  one  of  the  sources  of  light,  the  atoms  of 
oxygen  in  the  air  are  rushing  into  combination  with  the  atoms  (hy- 
drogen and  carbon)  of  the  burning  body  (344) ;  and  the  collision  of 
these  atoms  is  likely  to  set  them  vibrating.  These  vibrations  will  be 
communicated  to  the  atoms  of  the  surrounding  ether,  and  by  these 
transmitted  to  the  eye. 

FIG.  40. 


511.  Figure  40.— Diffraction  fringes  caused  by  inter- 
ference.— A  convenient  method  of  showing  diffraction  fringes  con- 
sists of  allowing  the  rays  of  the  sun  to  fall  on  the  flat  side  of  a  razor. 

The  rays,  S  and  T,  passing  in  close  proximity  to  the  back  and  edge 


288 


OPTICS. 


of  the  instrument,  will  be  deflected  as  represented.  A  portion  of  the 
rays  are  deflected  outward  as  if  reflected,  the  back  of  the  razor,  E,  de- 
flecting more  of  the  rays  outward,  and  the  edge,  F,  more  of  them  in-, 
ward.  If  the  body  be  narrow,  like  a  needle  or  hair,  Fig.  36  (505), 
the  rays  deflected  inward  cross  each  other  and  produce  interference,  in 
accordance  with  the  wave-theory.  The  rays  deflected  outward  produce 
interference  with  rays  not  deflected.  All  the  bright  and  dark  lines  are 
bordered  with  colored  fringes,  as  in  ordinary  cases  of  interference. 

If  a  beam  of  sunlight  pass  through  a  lens  in  a  dark  room,  and  fall 
upon  a  white  screen,  and  any  small  opaque  body  placed  in  the  light  so 
its  shadow  will  also  fall  on  the  screen,  the  shadow,  instead  of  being 
sharply  defined,  is  surrounded  by  three  colored  fringes,  the  outer  one 
being  very  faint.  If  homogeneous  light  be  employed,  instead  of  the 
fringes,  there  will  be  seen  bright  rings,  separated  by  dark  spaces — the 
breadth  of  the  rings  varying  with  the  color  of  light.  When  white 
light  is  used,  these  different  sets  of  colored  rings  blend,  producing  the 
fringes. 


Polarization  of  Light. 

512.  Poles  in  physics. — The  name  poles  is  given,  in  physics,  in 
general,  to  the  sides  or  ends  of  any  body  which  enjoy  or  have  acquired 
any  contrary  properties. 

513.  Figure  41.— Transmission  of  luminous  waves.— 

The  subject  of  polarized  light  constitutes  the  most  interesting  branch 

FIG.  41. 


of  optics.    The  scope  of  this  compendium,  however,  will  admit  of  only 
a  brief  explanation  of  a  few  of  its  leading  phenomena. 

In  connection  with  Fig.  33  (500),  it  was  shown  that  vibrations 
take  place  in  every  possible  direction  transverse  to  the  ray ;  but  for 
convenience  of  explanation,  we  will  suppose  they  take  place  only  in 
two  directions,  as  in  the  directions  of  the  horizontal  and  vertical  arrows 


OPTICS.  289 

(Fig.  33),  and  that  every  ray  consists  of  two  sets  of  colorless  undula- 
tions. 

Let  AE  (Fig.  41)  represent  the  plane  in  which  one  set  of  these  undu- 
lations takes  place ;  and  NH,  the  plane  in  which  the  other  set  of  waves 
occurs — the  planes  intersecting  each  other  at  right  angles. 

Let  K  represent  a  frame,  provided  with  fixed  cross-bars,  L ;  and  sup- 
pose the  planes  AE  and  HN  to  be  intersecting  pieces  of  card-board. 
If  now  an  eifort  be  made  to  thrust  these  card-boards  through  the  frame, 
K,  it  is  evident  that  while  one  slip  of  paper  will  pass,  the  other  will  be 
checked ;  but  if  the  frame,  K,  be  turned  one-fourth  round,  the  slip  of 
paper  which  was  before  checked  will  now  pass  through  the  bars,  L,  but 
the  one  which  passed  in  the  first  case  will  be  stopped. 

If,  instead  of  the  frame,  K,  we  take  a  thin  plate  of  a  certain  gem, 
called  the  tourmaline,  and,  instead  of  the  slips  of  paper,  the  ray  of  light 
composed  of  the  two  sets  of  undulations,  which,  as  we  have  supposed, 
vibrate  in  planes  at  right  angles  to  each  other,  it  will  be  found  that 
one  set  of  these  undulations  will  be  transmitted,  and  the  other  set 
intercepted,  when  the  tourmaline  is  held  in  one  position ;  but,  if  the 
tourmaline  be  turned  one-fourth  round,  the  rays  that  before  passed  are 
now  stopped,  and  those  that  in  the  first  case  were  intercepted  now  pass. 

When  light  has  been  thus  treated,  or  when,  by  any  means,  but  one  set 
of  undulations  is  obtained,  the  light  is  said  to  be  polarized. 

Opaque  bodies  allow  no  luminous  vibrations  to  pass  through  them. 
Some  bodies  transmit  nearly  all  the  luminous  vibrations  which  fall 
upon  them ;  while  other  bodies  are  capable  of  transmitting  only  those 
vibrations  of  light  which  move  in  a  single  plane,  or  those  undulations 
which  can  be  resolved  into  that  plane.  Other  bodies,  which  are  them- 
selves capable  of  vibrating  in  two  directions,  reduce  all  the  vibrations 
which  they  transmit  to  vibrations  in  the  two  planes  in  which  they 
themselves  vibrate.  Other  bodies  alter  the  direction  of  vibrations  of 
light,  which  fall  upon  them  at  certain  angles  of  incidence,  so  as  to 
transmit  vibrations  which  lie  in  a  single  plane. 

FIG.  42. 


514.  Figure  42.—  Action  of  tourmaline  on  ordinary 
light. — Let  EF  be  two  tourmaline  plates  symmetrically  held,  and  the 

19 


290  OPTICS. 

arrow,  A,  a  ray  of  light;  and,  as  shown,  the  ray  will  pass  through  both 
plates.  If  now  one  of  the  plates,  as  T,  be  turned  a  quarter  round,  as 
shown,  a  ray  of  light,  L,  will  pass  through  the  first  plate,  H,  as  before, 
but  not  through  the  second  plate,  T. 

If  the  light  which  has  been  transmitted  through  the  first  plate  be 
received  upon  a  plate  of  glass  at  an  angle  of  incidence  of  56°  45',  it 
will  be  wholly  reflected,  in  a  certain  position  of  the  glass,  and  wholly 
transmitted  if  the  glass  be  turned  round  through  90°. 

A  plate  of  tourmaline  affords  a  convenient  means  of  determining 
whether  a  ray  of  light  has  been  polarized  by  other  means. 

515.  Figure  43.— Polariscope.— Polarization  by  reflec- 
tion. —An  instrument  employed  for  polarization  of  light  by  reflection 
is  called  a  polar iscope.  If  the  light  of  the  candle  falls  upon  the  mir- 

FIG.  43. 


ror,  T,  making  the  angle  of  incidence  56°  45 ',  from  this  mirror  it  is 
reflected,  through  the  tube  AE,  to  the  mirror  F,  falling  upon  it  at  the 
same  angle  of  incidence,  56°  45',  and  is  thence  reflected  to  the  eye, 
at  L.  The  mirrors  are  at  right  angles  to  each  other,  and  the  candle  is 
hardly  perceptible  to  the  eye  at  L.  If  now  the  mirror,  F,  be  gradually 
turned,  by  revolving  the  tube,  E,  in  the  tube,  A,  the  image  of  the  can- 
dle grows  brighter  and  brighter,  and  is  the  brightest  possible  when  the 
planes  of  the  mirrors  are  parallel  to  each  other.  Hence,  from  the  posi- 
tion of  the  eye,  N,  the  image  is  perfect,  though  it  can  scarcely  be 
discerned  at  L. 

Light  is  polarized,  more  or  less,  by  reflection  from  many  different 
substances,  such  as  glass,  water,  air,  ebony,  mother-of-pearl,  surfaces  of 
crystals,  etc.,  provided  the  light  falls  at  a  certain  angle  peculiar  to  each 
surface,  called  the  polarizing  angle. 


OPTICS.  291 

516.  Plane  polarization. — When  light  has  been  polarized  so 
that  all  its  undulations  move  in  a  single  plane,  it  is  said  to  be  plane 
polarized. 

If  a  bundle  of  stretched  cords  of  different  sizes  were  vibrating  in  the 
same  direction,  it  would  represent  plane  polarized  light ;  and  the  differ- 
ence in  size  or  tension  of  these  cords  would  cause  a  difference  in  the 
length  of  their  waves;  hence  the  different  cords  may  represent  the 
different  colors,  which  also  differ  in  the  length  of  their  undulation. 

517.  Waves  in  any  number  of  planes  resolved  to  two 
planes. — If,  instead  of  two,  Fig.  41  (513),  there  are  an  infinite  num- 
ber of  planes  intersecting  each  other  in  the  manner  of  AE  and  NH, 
and  undulations  of  a  beam  of  light  are  passing  in  all  these  planes, 
these  undulations  can  all  be  resolved  to  two  planes,  which  shall  inter- 
sect each  other  at  any  required  angle.     When  resolved  to  two  planes, 
intersecting  each  other  at  right  angles,  the  sum  of  resulting  intensities 
in  the  one  plane  will  equal  the  sum  of  intensities  in  the  other.     A  ray 
of  ordinary  light,  therefore,  may  be  considered  as  consisting  of  undu- 
lations moving  in  two  planes  at  right  angles  to  each,  other. 

Any  medium  that  will,  either  by  its  position  or  molecular  constitu- 
tion, separate  light  into  two  parts,  undulating  in  planes  at  right  angles 
to  each  other,  will  produce  the  change  denominated  polarization  of 
light. 

518.  Partial  polarization  of  light.— Light  reflected  or  re- 
fracted at  any  oblique  angle,  is,  in  general,  partially  polarized ;  and  by 
repeated  reflections  and  refractions  the  degree  of  polarization  is  in- 
creased, until,  at  last,  it  is  apparently  completely  polarized. 

519.  Double  refraction  is  a  property  which  certain  transparent 
crystals  possess,  of  causing  a  ray  of  light,  in  passing  through  them,  to 
undergo  two  refractions ;  that  is,  the  single  ray  of  light  is  divided  into 
two  separate  rays,  causing  objects,  seen  through  such  a  .crystal,  to 
appear  double. 

A  common  mineral,  called  Iceland  spar,  which  is  a  crystallized  form 
of  carbonate  of  lime,  possesses,  to  a  remarkable  degree,  these  refracting 
properties.  The  form  of  this  crystal  is  that  of  a  rhomb,  or  rhom- 
boid. 

In  all  such  crystals  there  are  one  or  more  directions  along  which 
objects,  when  viewed  through  them,  appear  single :  these  directions  are 
termed  the  axes  of  double  refraction,  or  major  axes.  In  the  case  of 
Iceland  spar,  there  is  one  such  axis  which  joins  the  two  obtuse  three- 
sided  angles.  If  the  summits  of  these  angles  be  ground  down  and 


292  OPTICS. 

polished,  no  double  refraction  will  be  seen  through  the  crystal  in  this 
direction. 

One  of  the  refracted  rays  will  conform  to  the  law  of  ordinary  refrac- 
tion, and  is,  therefore,  called  the  ordinary  ray.  The  other  ray  does 
not  lie  in  the  same  plane  as  the  incident  and  ordinary  rays,  and  does 
not  conform  to  the  law  of  sines  (394)  ;  and,  therefore,  it  is  called  the 
extraordinary  ray. 

In  the  case  of  Iceland  spar,  the  index  of  refraction  for  the  ordinary  ray 
is  constantly  1.6543  ;  that  of  the  extraordinary  ray  varies,  being  1.4833, 
when  it  makes  an  angle  of  90°  with  the  major  axis. 

The  phenomenon  of  double  refraction  is  due  to  the  molecular  struc- 
ture of  the  medium  through  which  the  light  passes. 


Polarization  by  double  refraction.  —  When  light  is 
transmitted  through  a  double  refracting  substance,  both  the  ordinary 
and  extraordinary  rays  are  thereby  completely  polarized,  whatever  be 
the  color  of  the  light  employed.  The  tourmaline  plate,  or  other  an- 
alyzer, will  transmit  the  ordinary  image,  and  wholly  intercept  the 
other  ;  but  if  the  tourmaline  be  rotated  90°,  it  will  then  transmit  the 
extraordinary  and  intercept  the  ordinary  ray. 

S21.  Useful  applications  of  polarized  light.  —  Since  the 
discovery  of  polarized  light,  its  principles  have  been  applied  to  many 
practical  results. 

Thus,  it  has  been  found  that  all  reflected  light,  come  from  whence  it 
may,  acquires  certain  properties  by  which  it  can  be  distinguished  from 
direct  light. 

It  has  been  found  that  light  from  incandescent  bodies,  as  red-hot 
iron,  glass,  etc..  is  polarized  light;  but  that  light  from  an  inflamed 
gaseous  substance,  as  illuminating  gas,  is  always  in  a  natural  state,  or 
unpolarized.  Applying  these  principles  to  the  sun,  it  has  been  dis- 
covered that  the  light-giving  substance  of  the  sun  is  of  the  nature  of  a 
gas,  and  not  a  red-hot  solid  or  liquid  body. 

By  means  of  polarized  light,  the  chemist  can  detect  one-thirteen 
millionth  of  a  gramme  of  soda,  and  distinguish  it  from  potassa  or  any 
other  alkali. 

Polarized  light  is  found  to  be  of  great  value  in  various  microscopic 
investigations. 

Especially  important  is  such  light  in  physiological  chemistry,  as 
in  the  examination  of  crystals  found  in  various  cavities  and  fluids  of 
both  plants  and  animals. 

Polarized  light  is  of  great  importance  in  many  departments  of 
natural  science. 


OPTICS. 


Shadows. 


293 


522.  Figure  44.— Shadows  of  bodies  larger  than  the 
illuminating  body. — When  rays  of  light  radiate  from  a  luminous 
point  through  the  surrounding  space,  on  account  of  moving  in  straight 
lines  they  will  be  excluded  from  the  space  behind  the  body.  The 
comparative  darkness  thus  produced  is  called  a  shadow. 

When  the  luminous  body  is  smaller  than  the  opaque  body,  the  shadow 
of  the  opaque  body  will  gradually  increase  in  size  with  the  distance, 
without  limit.  Thus,  the  ball,  L,  being  larger  than  the  luminous 
point  of  the  candle  (supposing  the  light  of  the  candle  to  emanate  from 
a  point),  will  cast  a  shadow  upon  the  screen,  in  the  three  positions, 

FIG.  44. 


iiil! 


liiiOi! 


niiiiiir 


A,  E,  F,  of  different  sizes,  depending  upon  the  distance,  as  shown. 
The  shadows  on  the  screens  will  be  larger  the  nearer  the  ball  is  placed 
to  the  light. 

If  the  luminous  body  is  a  mere  point,  the  body  will  cast  a  well- 
defined  shadow  upon  the  screen.  If  either  of  the  straight  diverging 
lines  be  carried  around  the  sphere,  L,  touching  it  all  the  way,  it  will 
mark  the  exact  limits  of  the  shadow  cast  by  the  sphere,  which,  being 
round,  shows  that  light  moves  through  air  in  straight  lines. 

If  the  luminous  and  opaque  bodies  be  of  the  same  size,  the  shadow 
will  not  increase  or  diminish  ;  and  its  shape  will  be  cylindrical. 

FIG.  45. 


523.  Figure  45.— Shadows  of  bodies  smaller  than  the 
illuminating  body.— When  the  luminous  body,  8,  is  larger  than 


OPTICS. 


the  opaque  body,  E,  the  shadow  will  gradually  diminish  in  size  until 
it  terminates  in  a  point.  The  shape  of  the  shadow  of  a  spherical  body 
will  be  that  of  a  cone,  T.  The  length  of  the  cone  will  be  increased  by 
increasing  the  distance  between  the  luminous  and  opaque  body. 


an(i  penumbra  (Fig.  45).—  If  the  illuminating 
body  is  not  a  mere  point,  the  shadow  cast  will  have  an  indistinct  out- 
line, called  the  penumbra  ;  from  pene,  almost,  and  umbra,  a  shadow. 

S  represents  the  sun  ;  E,  the  earth  ;  T,  the  shadow  or  umbra  of  the 
earth  ;  L,  the  penumbra  of  the  earth.  If  the  line,  SE  (to  the  end  of 
the  shadow),  be  carried  around  the  earth  and  the  sun,  it  will  describe 
the  circumference  of  the  umbra,  T.  If  either  of  the  cross-lines  be 
carried  around  the  earth  and  sun,  it  will  describe  the  circumference  of 
the  penumbra,  L,  as  far  as  the  extent  of  the  shadow. 

The  breadth  of  the  penumbra  increases  with  the  diameter  of  the 
luminating  body,  and  with  the  distance  which  the  shadow  extends 
behind  the  opaque  body.  The  darkness  of  the  penumbra  gradually 
increases  from  the  borders  toward  the  umbra. 

525.  Figure  46.—  Density  of  shadows.  —  Shadows  are  of 
different  degrees  of  darkness,  because  the  light  from  other  luminous 

FIG.  46. 


bodies  (or  from  bodies  reflecting  light)  reaches  the  place  where  the 
shadow  is  formed.  This  is  shown  by  light  from  two  or  more  luminous 
points  falling  on  the  same  opaque  body. 

Let  A  be  an  opaque  body  illuminated  by  three  candles.     The  light  E 
will  produce  the  shadow  L  ;  the  light  F,  the  shadow  K ;  the  light  H,  the 


OPTICS.  295 

shadow  J.  But,  as  the  light  from  each  of  the  candles  shines  upon  all 
the  shadows  except  its  own,  the  shadows  will  all  be  faint. 

For  instance,  the  shadow  J  is  illuminated  by  the  candles  E  and  F, 
as  shown  by  the  dotted  lines.  If  the  candle  E  be  extinguished,  the 
shadow  L  will  disappear,  and  the  shadows  J  and  K  will  be  darker.  If 
the  candle  F  be  extinguished,  the  shadow  K  will  disappear,  and  the 
shadow  J  will  be  still  darker  and  well  defined. 

The  darkness  of  a  shadow,  when  it  is  produced  by  the  interruption 
of  the  rays  from  a  single  luminous  body,  is  proportioned  to  the  inten- 
sity of  the  light. 

The  forms  of  shadows  prove  that  light  moves  through  the  air  in 
straight  lines. 

526.  Figure  47.— Velocity  of  light. —Light  moves  with 
such  rapidity  that  its  movement  through  any  distance,  limited  by  the 
surface  of  the  earth,  cannot  be  appreciated  by  our  unaided  senses.  Its 
velocity  was  first  determined  about  two  hundred  years  ago,  by  the 
astronomer,  Roemer ;  who  observed  that  the  occurrence  of  the  eclipses 
of  Jupiter's  first  satellite  were  subject  to  certain  uniform  changes. 

Let  A  and  E  represent  the  earth  in  different  parts  of  its  orbit ;  J, 
Jupiter ;  F  and  L,  Jupiter's  first  moon.  The  direction  in  which  the 

FIG.  47. 


earth  rotates  around  the  sun  is  indicated  by  an  arrow,  as  also  the 
motion  of  the  satellite  around  the  planet. 

As  the  earth  moves  from  T,  its  nearest  position  to  Jupiter,  to  S,  its 
most  remote  position,  the  intervals  between  the  consecutive  eclipses  of 
the  satellite  gradually  grow  longer;  whilst  in  moving  from  S  back 
to  T,  these  intervals  grow  shorter.  The  total  retardation  in  passing 
from  T  to  S  is  nearly  16^  minutes,  and  just  equal  to  the  acceleration 
in  passing  from  S  back  to  T.  As  the  distance  from  T  to  S  is  190,000,000 
miles,  of  course,  the  velocity  of  light  (reducing  16J  minutes  to  seconds) 
equals  190,000,000  divided  by  990,  or  about  192,000  miles  per  second. 

At  F  the  satellite  is  just  emerging  from  the  shadow  of  the  planet, 
and  will  be  seen  from  the  earth  in  its  position  at  E ;  now,  while  the 


296 


OPTICS. 


satellite  passes  around  to  L  and  again  emerges  at  F,  the  earth  will 
have  moved  on,  say  to  some  point,  as  A ;  hence  the  light  from  the 
emerging  satellite  will  be  retarded  from  E  to  A  as  long  as  it  will  take 
for  light  to  pass  from  E  to  A,  and  so  on.  As  the  satellite  revolves 
around  the  planet  every  48  hours,  this  process  will  be  repeated  104 
times  while  the  earth  passes  from  T  to  S ;  and  104  times,  with  reverse 
effect,  while  it  moves  from  S  to  T. 

Velocity  of  light  is  ascertained  by  other  means  (see  509). 

The  mind  cannot  conceive  a  velocity  of  192,000  miles  per  second. 
Yet  it  takes  more  than  four  hours  for  the  light  of  Neptune  to  reach 
the  earth.  It  is  susceptible  of  proof,  that  light  is  three  years  in  coming 
from  the  nearest  fixed  star  to  the  earth ;  while  many  stars  have  been 
seen,  by  the  aid  of  instruments,  which  astronomers  infer  are  more  than 
a  thousand  times  as  far  from  us  as  the  nearest  one ;  requiring  more 
than  three  thousand  years  for  their  light  to  reach  the  earth,  notwith- 
standing its  inconceivable  velocity  of  192,000  miles  per  second.  How 
vast,  therefore,  must  be  our  universe.  Yet  all  this  system,  called  our 
cluster  of  stars,  is  but  a  small  part  of  the  Grand  Whole — the  Bound- 


527.  Figure  48.— Intensity  of  light.—The  intensity  of  light 
is  the  amount  of  disturbance  which  it  imparts  to  the  ether. 

The  illuminating  power  of  a  light  depends  upon  several  conditions : 

1.  As  the  distance  increases  it  becomes  less,  as  will  be  explained 
presently. 

2.  The  absolute  intensity  of  the  light  also  determines  the  result ; 
thus,  there  are  flames  that  are  very  brilliant  and  others  that  are  paler. 

3.  The  absorbent  effect  exerted  on  the  passing  rays  by  the  air,  or 
other  medium  traversed. 

4.  The  direct  or  oblique  manner  in  which  the  rays  are  received  on 
the  illuminated  surface. 

This  last  condition  is  illustrated  by  the  figure.     The  parallel  lines 
FlG  48  representing  a  given  number  of  rays 

of  light,  falling  upon  the  oblique  and 
vertical  mirrors,  M  and  N,  will  not 
illuminate  them  equally;  for,  all  the 
rays  that  fall  on  M  will  fall  on  N,  by 
removing  M ;  but  M  has  a  larger  sur- 
face than  N.  Hence,  the  greatest  illu- 
minating effect,  other  things  being 
equal,  will  be  realized  when  the  rays 
fall  upon  the  illuminated  body  perpen- 
dicular to  its  surface. 


OPTICS.  297 

Photometers. 

528.  Figure  49. — Photometers  are  instruments  used  to  meas- 
ure the  comparative  intensity  of  different  lights.  There  are  several 
kinds  of  these  instruments,  but  none  of  them  are  as  satisfactory  for 
measuring  the  intensity  of  lights  as  thermometers  are  for  measuring 
the  comparative  heat  of  bodies. 

Ritchie's  photometer  depends  on  the  equal  illumination  of  sur- 
faces. It  consists  of  a  box,  AA,  six  or  eight  inches  long,  by  one  inch 

FIG.  49. 


square,  in  the  middle  of  which  is  a  double  inclined  plane,  L,  which  is 
covered  with  white  paper,  neatly  doubled  to  a  sharp  edge  at  the  top  or 
angle.  In  the  top  of  the  box  is  a  conical  tube,  F,  at  the  upper  end  of 
which  the  eye  is  placed.  Place  the  two  lights,  the  comparative  inten- 
sities of  which  are  to  be  determined,  at  opposite  ends  of  the  box,  and 
the  reflected  light  of  each  will  be  seen  at  the  top  of  the  tube,  F,  as  rep- 
resented by  the  arrows.  Now  place  the  brighter  light  of  the  two  at 
such  a  distance  that  its  reflected  light  will  equal  that  of  the  other. 
Then,  measuring  their  distances  from  the  paper  on  the  inclined  planes, 
L,  their  illuminating  powers  are  as  the  squares  of  those  distances. 

Rumford's  photometer  depends  on  the  principle,  that  of  two 
lights,  the  more  brilliant  one  will  cast  the  deepest  shadow, — the 
brighter  light  being  removed  from  the  ground-glass  screen  until  the 
shadows  are  of  equal  density.  Then,  as  before,  their  relative  intensi- 
ties are  as  the  squares  of  the  distances  of  the  lights  from  the  shadows. 
A  partition  may  be  placed  between  the  lights. 

Silliman's  photometer  is  the  reverse  of  Rumford's,  comparing 
two  disks  of  light  thrown  up  by  two  equal  triangular  prisms,  upon  a 
disk  of  ground  glass  in  the  body  of  a  dark  chamber. 

Bunsen's  photometer  is  convenient,  and  consists  of  a  disk  of 
paper,  four  or  five  inches  in  diameter,  rendered  translucent  by  washing 


298  OPTICS. 

it  with  paraffine  dissolved  in  oil  of  turpentine,  except  a  small  part  of  it 
in  the  centre.  Place  this  disk  between  the  two  lights,  and  if  their 
intensities  are  unequal  the  translucent  part  can  be  distinguished  from 
the  central  part  of  the  paper ;  but  when  the  disk  is  placed  so  that  the 
two  parts  of  the  paper  appear  the  same,  the  disk  is  equally  illuminated 
by  the  two  lights,  for  which  reason  no  light  shines  through.  Of  course, 
the  relative  intensities  of  the  two  lights,  as  before,  will  be  as  the  squares 
of  their  distances  from  the  disk. 

529.  Figure  60.— Intensity  of  light  at  different  dis- 
tances.— It  can  be  shown,  mathematically,  that  the  intensity  of  light, 
coming  from  the  same  source,  varies  inversely  as  the  square  of  the  dis- 
tance from  its  source. 

If  a  board,  one  foot  square,  be  placed  one  foot  from  the  luminous 
point,  it  will  cast  a  shadow  that  will  cover  a  space  two  feet  square  at 

FIG.  50. 


double  the  distance,  and  three  feet  square  at  three  times  the  distance, 
and  so  on  (448).  The  areas  of  these  shadows  will  be  as  the  square  of 
their  distances  from  the  luminous  point,  or  as  I2,  23,  33,  etc.,  or  as 
1,  4,  9,  as  shown  in  the  diagram,  by  the  dotted  lines.  But  as  no  more 
light  would  occupy  the  space  at  3  feet  than  at  2  feet  or  at  1  foot  from 
the  luminous  point,  the  intensity  of  light  at  1,  2,  3,  etc.,  feet,  is  as  1, 
J,  |,  etc.,  or  as  9,  4,  1. 

Hence  it  is  seen  that  light  follows  the  same  law,  with  regard  to  its 
intensity  at  different  distances,  that  is  observed  for  gravity,  heat,  and 
sound. 


ACOUSTICS.  299 


CHAPTER    XII. 

(CHART  NO.  7.) 
ACOUSTICS. 

PRODUCTION    AND    PROPAGATION    OP    SOUND. 

530.  Definition. — Acoustics  (signifying  to  hear)  is  that  branch 
of  Physics  which  treats  of  the  nature,  phenomena,  and  laws  of  sound. 

531.  Sonorous  or  sounding:  bodies. — If  an  elastic  body,  for 
example,  a  glass  bell-jar,  held  by  the  knob,  be  struck  with  the  knuckle, 
its  particles  execute  a  series  of  tremulous  movements,  and  gradually 
return  to  a  position  of  rest.     Bodies  thus  capable  of  vibrating  are  said 
to  be  sonorous  bodies. 

53%.  Mediums. — A  medium  is  that  substance  which  intervenes 
between  the  sonorous  body  and  the  organ  of  hearing,  or  the  auditory 
nerve.  The  ordinary  medium  is  the  atmospheric  air. 

A  medium,  therefore,  transmits  the  vibrations  of  the  sonorous  body 
to  the  organ  of  hearing.  The  air  adjacent  to  the  sonorous  or  vibrating 
body  is  thrown  into  a  wave-like  motion,  and  this  movement  of  the  air 
is  communicated  to  the  air  next  beyond,  and  so  on,  until  the  sound- 
ivave  dashes  against  the  drum  of  the  ear ;  whence  it  is  transmitted,  by 
a  complex  mechanism,  to  the  auditory  nerve,  and  so  to  the  sensorium, 
or  seat  of  sensation.  In  rapid  successions,  these  condensations  and 
rarefactions,  or  sound-waves,  flow  from  the  sonorous  body  until  its 
vibrations  cease. 

Other  substances,  besides  air,  act  as  media,  such  as  wood,  water, 
iron,  etc. 

533 .  Sound  a  sensation. — Sound  is  the  sensation  produced  on 
the  organ  of  hearing,  by  the  vibrations  of  sonorous  bodies,  communi- 
cated by  undulations  or  sound-waves  of  intervening  media.  The  par- 
ticles of  the  medium  do  not  pass  from  the  sonorous  body  to  the  ear, 
but  only  the  undulations.  The  sonorous  body  may  exist  and  the  waves 
pass,  but  these  do  not  constitute  sound ;  the  sensation  which  they  pro- 
duce on  the  sensorium  is  what  is  called  sound. 


300  ACOUSTICS. 

534.  Different  sounds. — The  quality  of  sound,  or  the  character 
of  impression  on  the  auditory  nerve,  depends  upon  the  peculiarity  of 
the  waves  or  undulations  which  fall  upon  the  ear ;   and  these  will 
depend  upon  the  nature  of  the  sonorous  body,  and  the  character  of  the 
medium  or  media  through  which  the  vibrations  of  the  sonorous  body 
are  transmitted  to  the  ear. 

535.  Sonorous  difference  of  bodies. — The  quality  of  sono- 
rousness of  a  body  depends  upon  its  nature  and  molecular  structure, 
its  shape,  its  size,  etc.     For  example,  every  bell  will  differ  in  its  vibra- 
tions from  every  other  bell,  from  which  it  differs  either  in  composition, 
size,  or  shape.     Every  kind  of  wood  has  its  own  quality  of  sonorous- 
ness ;  and  a  piece  of  any  particular  kind  of  wood  will  vary  with  its 
shape,  size,  dryness,  etc.     The  same  is  true  of  different  metals  and 
other  solids. 

If  strings  be  stretched  between  two  fixed  points,  and  made  to  vibrate, 
the  quality  of  the  vibrations  will  vary  with  their  composition,  tension, 
length,  diameter,  etc. 

It  is  upon  this  difference  of  sonorousness,  depending  on  exact  condi- 
tions, that  we  are  enabled  to  construct  language,  speech,  music,  and,  in 
many  ways,  to  extend  scientific  investigation.  It  is  made  valuable  use 
of  in  the  investigation  of  certain  diseases  of  the  human  body.  If  the 
forefinger  be  placed  upon  the  body,  over  any  particular  organ,  as  the 
liver,  lungs,  heart,  etc.,  and  rapped  with  the  ends  of  the  fingers  of  the 
other  hand,  a  certain  sonorousness  will  be  perceived,  varying  in  differ- 
ent positions  on  the  body.  The  physician,  having  become  familiar 
with  these  different  sounds  for  the  healthy  and  for  the  unhealthy  states 
or  conditions  of  these  different  organs,  is  enabled,  in  any  given  case,  to 
determine  almost  the  precise  condition  of  an  internal  organ  by  this 
means,  known  as  percussion  (to  strike)  and  auscultation  (to  listen). 

53 6.  Time  is  required  for  the  transmission  of  sound. — 

The  blows  of  a  hammer  at  a  distance  are  heard  a  sensible  interval  of 
time  after  the  hammer  is  seen  to  fall.  The  flash  of  a  cannon  is  seen 
an  appreciable  time  before  the  report  is  heard,  though  the  gun  be  but 
a  little  distance  from  the  observer.  Thunder  is  heard  after  the  flash 
of  lightning  is  seen,  etc. 

537.  Calculation  of  distance  by  sound. — Knowing  the  ve- 
locity of  sound,  and  considering  that  of  light  to  be  instantaneous,  the 
distance  between  the  observer  and  the  sonorous  body  may  be  calculated 
by  observing,  as  in  the  case  of  the  cannon,  the  length  of  time  inter- 
vening between  the  flash  and  report. 


ACOUSTICS.  301 

Velocity  of  Sound. 

538.  The  velocity  of  all  sounds  is  the  same.—  The  ve- 

locity of  sound  is  the  space  that  it  traverses  in  a  second.  The  velocity 
of  the  vibrations  of  sonorous  bodies,  in  the  same  medium,  is  the  same 
for  all  sounds,  grave  or  sharp,  strong  or  feeble,  and  whatever  may  be 
their  pitch.  For  example,  there  is  no  confusion  in  the  effects  of  music, 
at  whatever  distance  it  may  be  heard. 

539.  Velocity  of  sound  in  air.  —  By  numerous  experiments,  it 
has  been  found  — 

1.  That  velocity  of  sound  decreases  with  the  temperature.    At  50°  F., 
it  is  1106  feet  per  second.     The  velocity  diminishes  about  one  foot  and 
a  tenth  for  every  degree  of  fall  of  temperature. 

2.  That,  at  the  same  temperature,  the  velocity  remains  the  same, 
whether  the  sky  is  bright  or  cloudy,  the  air  clear  or  foggy,  the  baro- 
metric pressure  great  or  small,  provided  the  air  is  tranquil.    The  inten- 
sity of  the  sound,  however,  as  it  falls  upon  the  ear,  is  more  or  less 
affected  by  all  these  conditions  (556). 

3.  That  the  velocity  varies  with  the  direction  and  velocity  of  the 
wind  (556). 


.  Velocity  of  sound  in  different  gases  and  vapors. 

—The  velocity  of  sound  in  the  different  gases,  is  in  the  inverse  ratio 
of  the  square  root  of  their  densities. 

At  the  temperature  of  32°  F.,  the  velocity  of  sound  in  carbonic  acid 
is  860  feet  per  second  ;  in  oxygen,  1040  feet  ;  in  air,  1092.54  feet  ;  in 
hydrogen,  4163  feet. 

541'  Velocity  of  sound  in  liquids.  —  Sound  is  transmitted 
through  liquids  as  well  as  through  gases.  Experiments  prove 
that  the  velocity  of  sound  in  water  is  4708  feet  per  second,  being  greater 
than  in  hydrogen  gas,  and  four  and  a  half  times  greater  than  in 
air. 

Agitation  of  the  liquids  does  not  affect  either  the  velocity  or  inten- 
sity of  the  sound.  But  the  interposition  of  solid  bodies,  as  walls,  etc., 
almost  destroys  the  sound  in  water  —  an  effect  which  does  not  take  place 
to  the  same  degree  in  air. 


Velocity  of  sound  in  solids.  —  Solid  bodies  transmit 
sound  with  much  greater  rapidity  than  gases  or  liquids  ;  but  the  velo- 
city is  not  equal  in  all  solids,  varying  with  their  elasticity,  density, 
homogeneity,  and  structure. 


302 


ACOUSTICS. 


Want  of  homogeneity  interferes  with  the  propagation  of  sonorous 
vibrations.  The  velocity  of  sound  in  iron  is  11,609  feet  per  second. 

It  would  require  nearly  three  years  for  sound  to  be  transmitted  by  an 
iron  rod  extending  from  the  sun  to  the  earth,  the  distance  that  light 
travels  in  eight  and  a  half  minutes.  In  wood  the  velocity  is  from  ten 
to  fifteen  times  greater  than  in  air. 


Time    required   to    distinguish    sounds.— The  ear 

cannot  distinguish  one  sound  from  another,  if  they  succeed  each  other 
at  an  interval  of  less  than  one-ninth  of  a  second. 


FIG.  i. 


REFLECTION     OF     SOUND. 

544-  Fi&ure  1.— Reflection  of  sound  at  right 
angles. — When  waves  of  sound,  or  rather  waves  of  air 
on  which  sound  is  borne,  impinge  on  a  solid  surface, 
they  are  reflected  from  it.  The  laws  regulating  reflec- 
tion of  sound  are  the  same  as  those  which  govern  the 
reflection  of  motion  (57),  and  heat  (291),  and  light  (368). 

The  waves  have  the  same  velocity  and  curvature  after 
as  before  reflection.  If  the  undulations  fall  upon  a  body 
in  the  direction  perpendicular  to  the  reflecting  surface, 
they  are  reflected  in  the  direction  of  the  incident  wave, 
as  indicated  by  the  arrow. 

545.  Figure  2.— Sounds  reflected  at  oblique  angles.— 

If  HL  be  the  direction  of  incident  waves,  upon  the  plane  surface  EF, 
.  2.  ^ne   reflected  waves   will   take   the 

direction  LT,  making  the  angle  of 
reflection,  NLT,  equal  to  the  angle 
of  incidence  NLH. 


546.    Circular   waves    re- 
flected  from    a    plane. — If   a 

circular  wave  fall  upon  a  plane  sur- 
face at  right  angles  to  it,  after  re- 
flection it  has  the  same  curvature, 
with  the  curve  reversed  ;  the  same  as  it  would  have  been  had  the  wave 
originated  from  a  point  on  the  opposite  side  of  the  plane,  and  as  far 
back  as  the  point  of  origin  itself  is  in  front  of  the  plane. 

547 ' •  Echoes. — An  echo  is  a  repetition  of  sound,  caused  by  reflec- 
tion of  the  sound-waves  from  an  obstacle,  as  a  rock  or  building,  more 


ACOUSTICS.  303 

or  less  remote.  Thus,  a  sound,  emanating  at  a  distance  from  a  hearer, 
is  heard  first  by  the  direct  or  original  undulations,  and  afterward  by 
the  reflected  waves. 

In  order  to  produce  an  echo,  therefore,  the  reflecting  body  must  be 
sufficiently  distant  from  the  source  of  sound,  to  make  the  time  be- 
tween the  arrival  of  the  original  and  reflected  waves  equal  to  or  greater 
than  one-ninth  of  a  second  (543) ;  otherwise  the  original  and  reflected 
sounds  will  blend  together,  and  produce  what  is  called  a  resonance,  and 
not  an  echo.  Hence,  in  small  rooms,  less  than  about  63  feet  across, 
there  can  be  no  echo ;  for,  as  sound-waves  move  at  the  rate,  say,  of 
1,125  feet  per  second  (539),  they  would  cross  the  room  and  return  to 
the  hearer  in  less  time  than  one-ninth  of  a  second,  if  the  walls  were 
nearer  to  each  other  than  about  63  feet ;  thus  producing  a  resonance 
(549),  instead  of  an  echo. 

At  this  distance,  only  the  echo  of  the  last  syllable  of  a  sentence  will 
be  heard.  If  the  distance  be  twice,  thrice,  etc.,  as  far,  then  there  will 
be  echoes  of  two,  three,  etc.,  of  the  last  syllables.  The  direct  sound 
and  reflected  sound  of  the  other  syllables  will  be  confounded  with  each 
other. 

548.  Figure  3. — Multiple  echoes. — The  same  sound  may  be 
reflected  from  several  objects  situated  in  different  directions  and  at 
different  distances,  producing  what  are  called  multiple  echoes. 

If  AH  and  BF  represent  two  parallel  walls,  and  a  sound  emanate  at 

FIG.  3. 


A,  it  will  radiate  in  all  directions  toward  the  opposite  wall.  In  pass- 
ing to  B,  it  will  be  reflected  back  to  A;  in  passing  to  T,  back  to  S, 
whence  it  will  be  again  reflected  to  F  and  back  again  to  H,  and  so  on. 
If  the  wall  BF  be  nearer  to  the  wall  AH,  as  in  the  position  of  the  line 
KL,  the  number  of  reflections  will  be  increased.  One  track  of  the 
waves,  in  this  case,  would  be  Al,  IN,  N2,  2S,  S3,  and  so  on. 

If  the  sound  emanate,  for  instance,  from  S,  it  would  be  reflected 
from  T  to  A  and  from  F  to  H  ;  or,  in  case  of  the  wall  being  at  KL, 
then  from  2  to  N,  K  to  1,  etc. ;  and  from  3  to  E,  E  to  4,  etc. 


304  ACOUSTICS. 

There  are  parallel  walls  which  are  said  to  repeat  sound  from  twenty 
to  thirty  times. 

Echoes  modify  the  tones  of  sound  ;  some  rendering  them  with  a  soft- 
ened, others  with  a  roughened  tone,  others  with  a  plaintive  accent,  etc. 

Eeflecting  surfaces  do  not  necessarily  require  to  be  hard  and  smooth, 
for  sounds  are  reflected  from  the  clouds ;  and  a  feeble  echo  occurs  even 
when  sound  passes  from  one  mass  of  air  to  another  of  different  density. 

5Jf9.  Resonance. — When  sounds  are  reflected  from  obstacles  at  a 
less  distance  than  about  63  feet,  or  echo-distance  (547),  the  reflected 
sound  is  superimposed  upon  the  direct  one,  thus  giving  rise  to  a  strength- 
ened sound,  which  is  called  resonance. 

It  is  easier  to  speak  in  a  closed  apartment  than  in  the  open  air,  in 
consequence  of  the  resonance  from  the  walls. 

The  resonance  is  more  perceptible  when  the  walls  are  plain  and 
elastic.  Hence,  it  is  more  clearly  perceived  in  rooms  devoid  of  furni- 
ture, draperies,  and  carpets. 

FIG.  4.  550.  Figure  4.— Sound  reflected 

in  a  sphere. — If  sound  were  to  emanate 
at  the  centre  of  a  hollow  sphere,  the  undu- 
lations would  reach  the  interior  surface  at 
all  points  at  the  same  time;  and,  falling 
on  the  surface  at  right  angles,  would  all 
be  reflected  back  to  the  centre  at  the  same 
time,  causing  a  concentration  of  echo  or 
resonance,  depending  upon  the  size  of  the 
sphere. 

551.  Figure   5. — Sound  propagated  from    the  foci  of 
an  ellipse. — If  the  figure  be  an  ellipse  and  a  wave  emanate  from  F, 
FlG    5  one  of  the  foci,  all  the  rays  will 

converge,  so  as  to  fall  simultane- 
ously, after  reflection,  at  the 
other  focus  L,  as  shown  by  the 
lines.  This  is  because  the  an- 
gles of  incidence  are  equal  to 
those  of  reflection,  from  focus 
to  focus. 

552.  Whispering  galle- 
ries are  so  called  because  a  low  whisper  uttered  in  one  point  in  them 
may  be  heard  distinctly  at  another  and  distant  point,  while  it  is  inau- 
dible in  all  other  positions. 


ACOUSTICS.  305 

Such  galleries  are  of  ellipsoidal  shape ;  and  the  whispering  takes 
place  at  one  focus  and  is  heard  at  the  other.  The  reason  the  whisper- 
ing cannot  be  heard  at  other  points,  is  because  there  is  no  other  point 
where  the  rays  converge.  For  this  reason,  too,  the  whispering  cannot 
be  heard  unless  it  takes  place  in  one  of  the  foci. 

553 .  Audience  rooms. — In  the  construction  of  public  rooms 
for  the  purpose  of  speaking,  such  forms  should  be  avoided  as  produce 
echoes  and  reverberation,  which  impair  the  distinctness  with  which  the 
speaker  is  heard. 

By  an  elaborate  series  of  experiments  and  observations,  it  is  found 
that  the  best  form  for  an  audience  room  is  one  shaped  like  a  fan,  the 
breadth  in  front  of  the  speaker  being  sixty-four  feet,  and  the  length 
one  hundred  feet.  The  height  of  the  room  should  not  exceed  thirty  or 
thirty-five  feet. 

554-  Figure  6.— Reflection  of  waves  by  parabolic  curves. 

— The  nature  of  a  parabolic  curve,  as  previously  shown  (392),  is  such 
that  heat,  light,  and  sound  proceeding  from  its  focus  will  be  reflected, 
by  the  curve,  in  parallel  lines;  or,  conversely,  parallel  rays,  falling 
upon  the  curve,  will  be  reflected  to  the  focus. 

This  is  proved  by  the  fact,  that  if  two  such  curves  be  placed  oppo- 
site to,  and  several  yards  from,  each  other,  as  represented,  and  a  watch 

FIG.  6. 


be  made  to  tick  in  the  focus,  F,  of  one,  it  will  be  heard  in  the  focus,  L, 
of  the  other,  although  it  tick  so  faintly  that  it  cannot  be  heard  at  any 
other  point  between  them,  even  if  the  ear  be  placed  quite  near  the 
watch. 

This  proves,  again,  that  the  angles  of  incidence  and  reflection  are 
equal. 

Intensity  of  Sound. 

555.  The  Intensity  of  sound  is  its  loudness ;  which  depends 
upon  the  amplitude  of  the  waves ;  which,  in  turn,  depends  upon  the 
force  or  amplitude  of  the  vibrations  of  the  sounding  or  sonorous  body. 

20 


306  ACOUSTICS. 

556.  Causes  which  modify  the  intensity  of  sound.— The 

following  are  some  of  the  causes  which  modify  the  intensity  of  sound : 

1.  It  is  shown  by  theory  and  experiment  that  the  intensity  of  sound 
at  different  distances  is  subject  to  the  same  law  which  governs  heat  and 
light  and  gravity.     That  is,  the  intensity  of  sound  varies  inversely  as 
the  square  of  the  distance  from  the  sonorous  body. 

2.  The  intensity  of  sound  diminishes  with  the  amplitude  of  the  vibra- 
tion of  the  aerial  particles.     This  will  be  appreciated  by  looking  at  the 
vibrations  of  a  musical  cord  or  a  tuning-fork,  and  observing  that  the 
sound  grows  fainter  as  the  amplitude  of  the  vibrations  diminishes. 

3.  Sound  is  modified  by  the  density  of  the  air.     If  the  air  is  rarefied, 
the  intensity  is  diminished,  as  shown  by  ringing  a  bell  in  the  exhausted 
receiver,  Fig.  23  (575).     Hence,  as  a  diminution  of  heat  increases  the 
density  of  air,  sounds  are  louder  in  cold  than  warm  weather. 

4.  Watery  vapor  being  a  good  conductor  of  sound,  its  presence  in  the 
air  increases  the  intensity  of  sounds. 

5.  The  wind  modifies  sound.     The  effect  of  wind  is  to  move  the  whole 
mass  of  air,  carrying  along  the  sound-waves  unaltered.      Hence  the 
velocity  of  sound  is  increased  or  diminished  by  the  velocity  of  the 
wind,  according  as  the  direction  of  the  wind  corresponds  with  or  is 
opposed  to  the  direction  of  the  sound. 

6.  The  intensity  of  sound  is  increased  if  the  sonorous  body  is  in  con- 
tact with  or  not  far  from  another  body,  capable  of  vibrating  in  unison 
with  it. 

It  is  upon  this  principle  that  sounding-boards  are  employed  in  musi- 
cal instruments,  as  the  piano,  etc.  In  the  case  of  the  violin,  the  air  in 
the  body  of  the  instrument  vibrates  in  unison  with  the  cords  or  strings. 

557 .  Intensity  of  sounds  in  tubes.— If  the  sound-waves  are 
prevented  from  spreading  in  all  directions,  the  particles  of  air  lose  but 
little  of  their  motion,  and  the  sound  but  little  of  its  intensity.     Hence 
the  employment  of  speaking  tubes,  through  which  conversation  can  be 
conducted  in  a  low  tone  of  voice  by  persons  situated  a  mile  from  each 
other. 

This  will  be  understood  by  referring  to  Fig.  3  (548).  The  wave  or 
sound,  AT,  is  more  intense  at  1  than  at  T,  and  more  intense  at  T  than 
it  would  be  further  on  in  its  direct  course.  Hence  the  intensity  at  S 
is  greater  when  the  wave  takes  the  track,  Al,  IN,  N2,  2S,  than  when 
it  takes  the  track  AT,  TS. 

558.  Figure  7. — The  ear-trumpet.— This  is  an  instrument 
employed  to  intensify  sound,  to  assist  persons  who  are  hard  of  hearing. 


ACOUSTICS. 


307 


The  mouth,  M,  of  the  instrument  is  turned  in  any  convenient  direc- 
tion, and  the  small  end,  N,  is  placed  in  the  ear. 

It  was  formerly  supposed  that  the  advantage  of  the  instrument  was 
due  to  reflection  of  the  sound  within  the  trumpet,  in  such  a  manner 
as  to  converge  the  undulations  and  direct  them  to  a  focus  at  the  point 
of  contact  between  the  instrument  and  the  ear,  as  illustrated  by  the 

FIG.  7. 


lines ;  but  it  has  been  found  that  the  instrument  does  not  operate  upon 
this  principle.  Its  advantage  is  due,  1st,  to  the  principle  explained  in 
the  previous  article,  and  2d,  to  its  wide  mouth  and  conical  shape,  irre- 
spective of  its  otherwise  exact  form,  by  which  the  portions  of  com- 
pressed or  dilated  air,  which  arrive  at  the  exterior  opening,  transmit 
their  compressions  or  dilatations  to  portions  of  air  smaller  and  smaller, 
and  consequently  transmit  them  with  increased  intensity. 

The  form  of  the  external  ear  of  animals  favors  the  collection  of  sound 
in  the  same  manner. 

559.  Figure  8. — Speaking-trumpet. — This  is  an  instrument 
employed  to  intensify  the  voice,  that  it  may  be  heard  in  the  midst  of 
other  sounds,  and  also  for  conveying  the  voice  to  a  great  distance. 

FIG.  8. 


The  instrument  is  conical,  terminated  by  a  bell-shaped  extremity,  M. 
and  provided  with  a  suitable  mouth-piece,  L. 


308 


ACOUSTICS. 


FIG.  9. 


As  in  the  case  of  the  ear-trumpet,  it  was  formerly  supposed  the  ad- 
vantage of  the  speaking-trumpet  was  due  to  the  reflection  of  the  undu- 
lations, in  such  a  manner  that  they  issued  in  the  direction  of  the  axis 
of  the  instrument,  as  represented  by  the  dotted  lines.  It  has  been 
shown,  however,  that  the  efficiency  of  the  instrument  is  not  due  to 
reflection  of  sound  from  its  walls,  but  simply  to  the  greater  intensity 
of  the  pulsations  produced  in  the  column  of  confined  air,  which  vibrate 
in  unison  with  the  voice  at  the  mouthpiece. 

560.  Figure  9. — Vibrations   of  sonorous  bodies  illus- 
trated by  the  Jews-harp. — This  little   instrument,  familiar  to 

every  one,  affords  a  convenient  illus- 
tration of  the  vibrations  of  sono- 
rous bodies.  If  its  tongue  be  struck 
with  the  finger,  its  vibrations  can 
be  distinctly  seen. 

The  different  sounds  given  out 
by  this  instrument,  when  in  use, 
depend  upon  the  variation  of  the 
currents  of  air  blown  across  its 
tongue  by  the  player,  and  upon 
varying  the  relative  position  of  the  lips  and  instrument  (585). 

561.  Figure  10.— Sound-waves   caused   by  striking  a 
bell. — The  vibrating  bell  causes  the  air  to  be  thrown  into  waves  of 

FIG.  10. 


condensation  and  rarefaction.  The  rarefactions  are  shown  by  the 
darker  portions  of  the  figure,  and  the  condensations  by  the  lighter  por- 
tions (567). 

562.  Figure  11. — The  cause  of  vibrations  in  sonorous 
bodies  illustrated  by  a  bell. — Let  the  dotted  circle  represent 


ACOUSTICS. 


309 


the  rim  of  a  bell  at  rest.     If  the  rim  of  FIG.  11. 

the  bell  be  struck  with  a  hammer,  it  is 
thrown  out  of  the  circular  shape  into  the 
form  of  an  ellipse,  shown  by  either  of  the 
elliptical  curves.  Now,  as  the  bell  is  an 
elastic  body,  it  will  spring  back,  not  only 
to  its  circular  form,  but  to  the  form  of  an 
ellipse  situated  at  right  angles  to.  the 
first  ellipse,  and  so  011 ;  alternately  ap- 
proaching and  receding  from  the  posi- 
tion of  equilibrium,  each  vibration  dimin- 
ishing in  amplitude,  until  all  parts  of  the  bell  come  again  to  a  state  of 
rest. 

It  is  the  springing  of  the  bell  forward  and  backward,  in  this  manner, 
that  propagates  the  undulations  shown  in  the  previous  figure. 

As  the  bell  springs  forward,  striking  the  air,  a  wave  of  condensation 
is  produced,  and  as  it  recedes  from  the  air,  a  wave  of  rarefaction  is 
produced  (567). 

563.  Figure  12. — Harmonicon. — This  is  a  musical  instru- 
ment, consisting  of  a  number  of  glass  goblets  of  different  sizes,  fastened 

FIG.  12. 


to  the  bottom  of  a  box  which  acts  as  a  sounding-board,  and  so  attuned 
to  each  other  as  to  form  the  harmonical  scale.  The  glasses  are  made 
to  vibrate  by  touching  the  edges  with  the  wet  finger,  and  their  tones 
may  be  prolonged,  and  made  to  swell  or  diminish,  like  those  of  the 
violin.  This  simple  contrivance,  first  invented  by  Franklin,  affords 
music,  which  for  sweetness,  delicacy,  and  smoothness,  is  hardly  sur- 
passed by  that  of  any  other  instrument. 


310  ACOUSTICS. 

Interference  of  Sound. 

564-  Figure  13. — Interference  of  sound. — If  two  series  of 
sonorous  undulations  encounter  each  other  in  opposite  phases  of  vibra- 
FlG  13  tion,  the  phenomena  of  interference  will   be   pro- 

duced. 

If  both  arms  of  the  tuning-fork  are  vibrating, 
they  will  recede  from  and  approach  each  other,  as 
indicated  by  the  dotted  lines.  If  the  instrument  be 
placed  about  a  foot  from  the  ear,  with  the  branches 
equidistant,  both  sounds  will  be  heard,  for  the  waves 
combine  their  effects.  Bat,  if  the  fork  be  slowly 
turned  around,  the  sound  will  grow  more  and  more 
faint,  until  a  position  will  be  reached  in  which, 
owing  to  interference,  total  silence  will  result.  If, 
however,  one  of  the  arms  ceases  to  vibrate  the  other 
will  be  heard.  See  interference  of  light  (503). 

565.  Combination  of  waves  of  liquids. 

— Combination  and  interference  of  waves  are  of  uni- 
versal occurrence  in  all  media  in  which  force  of  any  kind  is  propagated 
by  undulation. 

Two  systems  of  waves  encountering  each  other,  several  effects  may 
follow. 

1st.  If  the  elevations  of  two  waves  coincide,  and,  consequently,  their 
depressions  also,  then  a  new  wave  will  be  formed,  whose  elevation  and 
depression  will  be  the  sum  of  those  of  the  originals.  In  case  of  an 
elastic  fluid  and  sound-waves,  the  sound  at  this  point  would  be  louder. 

3d.  If  the  two  waves  are  of  equal  amplitude,  and  so  superimposed 
that  the  elevation  of  one  falls  into  the  depression  of  the  other,  then 
both  waves  disappear,  and  the  surface  remains  horizontal.  This  con- 
stitutes interference.  In  case  of  an  elastic  fluid  and  sound-waves, 
silence  would  occur  at  this  point. 

3d.  When  one  wave  has  greater  amplitude  than  the  other,  if  they 
meet  in  the  same  phase,  the  resulting  wave  will  have  a  height  equal  to 
the  difference  between  them.  In  case  of  an  elastic  fluid  and  sound- 
waves, partial  silence  would  occur  at  this  point. 

566.  Figure  14. — Interference  in  an  ellipse. — If  the  figure 
represent  an  elliptical  dish  of  water,  and  a  system  of  waves  be  formed 
about  each  of  the  foci,  the  two  sets  of  waves  will  encounter  each  other, 
as  represented  by  the  several  circles,  and  exhibit  the  phenomena  of 
interference. 


ACOUSTICS. 


311 


If  the  heavy  lines  are  the  elevations  and  the  lighter  lines  are  the  de- 
pressions, then  the  points  where  the  heavy  and  light  lines  intersect  are 

FIG.  14. 


points  where  an  elevation  coincides  with  a  depression,  which,  therefore, 
are  points  of  interference. 

At  these  points,  in  case  of  an  elastic  fluid,  there  would  be  silence,  if 
the  waves  were  those  of  sound. 

567.  Waves  of  condensation  and  rarefaction. — The  un- 
dulations of  liquids,  described  in  the  last  two  articles,  are  surface  waves, 
and  undulations  of  the  same  kind  may  be  produced  in  elastic  fluids. 
But  ivaves  of  condensation  and  waves  of  rarefaction  are  of  a  different 
character,  and  peculiar  to  elastic  fluids. 

Such  waves  are  produced,  in  air  and  gases,  by  any  disturbance  of 
density.  If  the  elastic  fluid  be  compressed,  and  again  suddenly  relieved 
from  compression,  it  will  expand,  and  in  its  expansion  exceed  its 
former  volume  to  a  certain  extent;  after  which  it  will  contract  and 
expand,  and  thus  oscillate  alternately  on  either  side  of  the  position  of 
repose  (561-2). 

568.  Interference    of    sound-waves  (Figs.  20  and  21).— 
Two  sets  of  undulations,  represented  by  the  two  curved  lines,  TS  and 
LN,  Fig.  20  (p.  315),  would  interfere  and  produce  silence,  as  at  i,  be- 
cause their  phases  are  so  related  to  each  other  that  the  depressions  of 
one  set  correspond  with  the  elevations  of  the  other. 


312  ACOUSTICS. 

If  a  vibrating  tuning-fork,  Fig.  21  (p.  315),  be  held  over  the  mouth 
of  a  cylindrical  glass  vessel,  E,  the  air  within  the  vessel  will  assume 
sonorous  vibrations,  and  a  tone  will  be  produced.  If  now  a  second 
glass  cylinder,  A,  be  held  in  the  position  shown,  the  musical  tone  pre- 
viously heard  will  cease ;  but  if  either  cylinder  be  removed,  the  sound 
will  be  renewed  again.  The  silence  is  caused  by  interference  of  the 
two  sets  of  sonorous  waves. 

Co-existence  of  sonorous  -waves. — Many  sounds  may  be  trans- 
mitted through  the  air  simultaneously.  In  listening  to  a  concert  of 
instruments,  a  practiced  ear  can  detect  the  particular  sound  of  each 
instrument ;  which  shows  that  the  sound-waves  cross  each  other  with- 
out modification,  notwithstanding  the  effects  of  interference,  as  pre- 
viously explained. 

569*  Undulation  of  solids. — Solid  bodies  exhibit  the  phenom- 
ena of  vibration  in  various  forms  and  degrees,  according  to  the  form 
of  the  body  and  the  manner  of  applying  the  force. 

Linear  bodies,  as  tense  wires,  strings,  etc.,  are  susceptible  of  three 
kinds  of  vibrations,  called  transverse,  longitudinal,  and  torsional. 

Vibration  of  Cords. 

570.  Figure  15. — The  elasticity  of  cords  and  -wires  is 
developed  by  tension. — If  a  cord,  TL,  be  stretched,  and  secured 
at  each  end,  and  then  drawn  out  in  the  middle  from  its  position  of 

FIG.  15. 


equilibrium,  as  shown,  upon  being  let  go,  its  elasticity  causes  it  to  re- 
turn to  its  former  position  with  accelerated  velocity,  which  carries  it 
past  the  position  of  equilibrium,  to  some  position  from  which  it  re- 
turns ;  and  again  passes  the  central  position,  and  so  on ;  until,  after  a 
great  number  of  oscillations,  it  at  length  comes  to  rest. 

These  oscillations,  at  first,  will  be  manifest  to  the  eye,  if  the  string 
be  of  considerable  length. 

One  complete  movement  from  side  to  side,  is  termed  an  oscillation 
or  vibration ;  and  the  time  occupied  in  performing  it,  is  called  the 
time  of  oscillation. 

L  and  T  are  thumb-screws  for  tightening  the  cord. 


ACOUSTICS. 


313 


571.  Figure  16.— Nodal  points  of  vibrating  cords.— If 

the  vibrating  cord  (Fig.  15)  be  touched  in  the  middle,  its  vibrations 
will  assume  the  form  shown  by  the  dotted  line  in  this  figure.    FE  will 

FIG.  16. 


equal  AE;  the  elevation,/,  will  equal  the  depression,  Z;  and  that  point 
of  the  cord.  E,  where  the  phases  of  elevation  and  depression  intersect, 
will  be  at  rest.  A  piece  of  paper  placed  on  this  point  will  rest  undis- 
turbed, while  it  would  be  thrown  off  of  any  other  part  of  the  cord. 
This  is  called  a  nodal  point  (from  the  Latin,  nodus,  a  knot). 

Figures  17  and  18.— Two  or  more  nodal  points  in  one 
string. — If  the  cord  be  touched  at  two  points,  dividing  the  string  into 

FIG.  17. 


three  equal  parts,  the  vibrations  will  assume  the  form  shown  by  the 
dotted  line  in  Fig.  17. 

If  the  cord,  HN",  Fig.  18,  be  touched  at  its  centre,  L,  and  at  S, 
midway  from  N  to  L,  the  vibrations  will  assume  the  form  represented 
by  the  dotted  lines. 

FIG.  18. 


Any  number  of  nodal  points  may  exist  in  the  same  string,  but  rarely 
more  than  four,  when  they  spontaneously  occur. 

572.  Laws  of  the  vibration  of  cords. — The  number  of  vibra- 
tions of  a  stretched  cord,  in  any  given  time,  as  in  one  second,  depends 
upon  its  length,  thickness,  tension,  and  density. 

Calculation  and  experiment  have  demonstrated  that  cords  vibrate  in 
accordance  with  the  following  laws. 

1.  The  tension  being  the  same,  the  number  of  vibrations  varies  inverse- 
ly as  its  length. 


314 


ACOUSTICS. 


This  property  is  utilized  in  the  violin.  By  applying  the  finger,  the 
length  of  the  vibrating  portion  of  the  cord  is  reduced  at  pleasure. 

2.  The  tension  and  length  being  the  same,  the  number  of  vibrations 
varies  inversely  as  its  size  or  thickness. 

A  cord,  therefore,  of  any  given  size,  makes  twice  as  many  vibrations 
as  one  of  double  the  size.  Other  things  being  equal,  the  notes  ren- 
dered differ  by  an  octave. 

3.  The  length  and  size  being  the  same,  the  number  of  vibrations  varies 
as  the  square  root  of  the  tension. 

Hence,  a  cord,  which  renders  a  given  note,  will,  if  its  tension  be 
quadrupled,  render  a  note  an  octave  higher,  and  so  on. 

4.  Other  things  being  equal,  the  number  of  vibrations  varies  inverse- 
ly as  the  square  root  of  the  density. 

Hence,  dense  cords  render  graver  notes  than  those  of  less  density. 
Large,  dense,  and  long  cords,  not  tensely  stretched,  give  grave  notes ; 
while  small,  light,  and  short  cords,  tensely  stretched,  yield  acute  notes. 

573.  Figure  19.— Verification  of  the  laws  of  vibra- 
tion.— The  sonometer. — The  laws  just  enunciated  are  verified  by 
means  of  an  instrument  called  a  sonometer,  or  sound-measurer.  This 
instrument  consists  of  a  wooden  box  about  four  feet  long,  upon  which 

FIG.  19. 


are  mounted  two  fixed  bridges,  F  and  E,  and  one  movable  bridge  H. 
Passing  over  the  fixed  bridges  are  two  cords,  ff  and  II.  One  end  of 
each  cord  is  fastened  to  the  box,  and  the  other  ends,  after  passing  over 
pulleys,  are  drawn  down  with  equal  force,  by  means  of  equal  weights, 
W,  as  shown.  On  the  edge  of  the  box  is  a  graduated  scale.  The 


ACOUSTICS. 


315 


length  of  the  vibrating  part  of  the  cords  will  depend  upon  the  position 
of  the  bridges. 

If  the  movable  bridge,  H,  is  placed  so  that  the  distance,  FH,  is 
equal  to  half  the  distance,  FE,  the  notes  of  the  two  cords  will  differ  by 
an  octave ;  that  is,  II  will  vibrate  twice  as  fast  as  ff.  If,  by  moving 
the  bridge  H,  FH  be  made  equal  to  one-third  of  FE,  then  II  will 
vibrate  three  times  as  fast  as  //.  This  verifies  the  first  law. 

Remove  the  bridge,  H,  and  substitute  two  other  cords  (of  the  same 
material),  one  of  which  is  twice  as  large  as  the  other ;  and  it  will  be 
found  that  the  notes  differ  by  an  octave.  If  one  cord  be  three  times 
as  large  as  the  other,  the  smaller  cord  will  vibrate  three  times  as  fast 
as  the  other ;  which  verifies  the  second  law. 

If  the  cords  are  every  way  alike,  and  one  is  stretched  by  a  weight 
four  times  as  great  as  that  used  to  stretch  the  other,  the  notes  will  differ 
one  octave.  If  the  weights  are  as  1  to  9,  the  rapidity  of  the  vibrations 
will  be  as  1  to  3.  This  verifies  the  third  law. 

If  the  cords  are  of  different  densities,  but  every  other  way  alike  and 
equally  stretched,  it  will  FlG 

be  found  that  the  fourth 
law  is  verified  in  each  case. 

Figure  20.— Inter- 
ference of  sound  il- 
lustrated by  two  vi- 
brating cords.  —  For 

explanation  of  this  figure, 
see  568. 

Figure  21.— Interference  of  sound  further  illustrated, 

by  means  of  a  common  tuning-fork  and  two  cylindrical  glass  vessels. 
For  the  explanation,  see  568. 

FIG.  21. 


316 


ACOUSTICS. 


FIG.  22. 


574-  Figure  22.— Sounds  caused  by  burn- 
ing hydrogen. — When  a  small  jet  of  hydrogen  is 
burned  within  a  glass  tube  of  about  an  inch  in  diame- 
ter, as  shown,  pleasant  musical  tones  are  heard,  which 
are  varied  by  raising  the  tube  up  and  down. 

The  vibrations  and  sounds  are  due  to  the  successive 
explosions  of  small  portions  of  free  gas,  mingled  with 
common  air.  The  ascending  current  of  air,  caused  by 
the  heat,  momentarily  extinguishes  the  flame,  permit- 
ting the  mixture  of  the  air  with  the  inflammable  gas. 
The  expiring  flame  kindles  this  explosive  mixture  and 
relights  the  jet.  These  successive  phenomena  occur 
with  great  rapidity  and  at  regular  intervals,  producing 
the  musical  note. 

The  hydrogen  may  be  generated  by  the  action  of 
dilute  sulphuric  acid  on  zinc,  placed  in  a  common 
bottle.  It  is  better,  however,  to  regulate  the  flow  of  the 
gas  to  the  tube  by  means  of  a  faucet. 


575.  Figure  23. — Sound  is  not  propagated  in  a  vacuum. 

-That  some  medium  is  necessary  for  the  transmission  of  sound  may 

be  shown  by  experiments  with  the 
exhausted  receiver. 

In  the  top  of  the  receiver  is  a  rub- 
ber plug,  L,  fitted  air-tight.  Extend- 
ing through  the  plug  is  a  rod,  upon 
the  end  of  which,  within  the  receiver, 
is  a  bell.  The  rod  is  bent  at  right 
angles  above,  to  form  a  handle,  T, 
with  which  to  ring  the  bell. 

The  sound  of  the  bell  can  be  dis- 
tinctly heard  when  the  receiver  is 
filled  with  air.  If  the  air  be  ex- 
hausted, the  bell  cannot  be  heard. 
By  exhausting  the  air  and  ringing  the  bell  at  the  same  time,  the  sound 
of  the  bell  grows  fainter  and  fainter,  until  it  ceases.  Hence,  sounds  at 
high  altitudes,  as  on  high  mountains,  are  not  so  loud  as  at  the  level  of 
the  sea. 


FIG. 


Vibrations  of  Bods  and  Plates. 

576.  Vibrations  of  rods. — Rods,  like  cords,  vibrate.  If  they 
are  fixed  firmly  by  one  of  their  extremities,  as  in  a  vice,  they  will,  when 
set  in  motion,  be  divided  by  stationary  undulations  into  several  vibrat- 


ACOUSTICS.  317 

ing  parts.  The  nodal  points  may  be  ascertained  by  placing  upon  the 
rods  light  rings  of  paper;  which  will  be  thrown  off  all  along  the  rod, 
except  at  the  nodal  points,  where  they  will  remain  unmoved. 

The  space  between  the  free  extremity  and  the  first  nodal  point  is 
equal  to  half  the  length  contained  between  two  nodal  points. 

577.  Means  of  vibrating  plates.  —  Vibrations  are   readily 
excited  in  elastic  plates  by  friction  or  blows,  and  sounds  are  evolved. 
The  plate  is  confined  either  at  its  centre  or  one  corner,  in  a  vice 
(Fig.  26),  resting  on  a  cone  of  cork  and  pressed  by  a  screw,  also  tipped 
with  cork,  as  represented. 

578.  Nodal  lines  of  plates. — In  the  vibration  of  plates  nodal 
lines  will  be  formed,  which  do  not  participate  in  the  movements  of  the 
plane,  but  remain  in  a  state  of  rest. 

579.  Determination  of  nodal  lines  of  plates.— The  posi- 
tion of  the  nodal  lines  may  be  determined  by  scattering  sand  or  other 
fine  material  over  the  plate,  and  causing  the  plate  to  vibrate,  as  by 
means  of  a  violin-bow  drawn  across  the  edge.     The  grains  of  sand  will 
be  thrown  from  the  vibrating  portions  of  the  plate,  and  come  to  a  state 
of  rest  on -the  nodal  lines  and  points. 

580.  Nodal  figures.— These  always  have  symmetry  of  form.    A 
great  variety  of  these  have  been  determined.     The  same  plate  may 
furnish  an  infinite  number  of  them,  which  pass  from  one  to  another 
in  a  continuous  manner,  and  not  by  sudden  changes. 

A  few  of  these  figures  are 
represented,  in  order  to  give  a 
general  idea  respecting  their 
formation. 

Figure  24.  — If  a  square 
plate  of  glass  be  grasped  in  the 
centre  by  the  hand-vice,  and 
sand  scattered  over  its  surface, 
and  the  violin-bow  drawn  rap- 
idly across  it,  close  to  one  of  its 
angles,  the  sand  will  be  thrown 
into  the  position  shown  by  the 
dots. 

Powdered  litmus,  previously  mixed  with  gum-water,  dried  and  pul- 
verized to  a  uniform  size,  may  be  used  instead  of  sand.  Figures  thus 
made  can  be  transferred  to  paper,  simply  by  moistening  the  paper 
with  gum-water,  and  pressing  it  upon  the  plate. 


318 


ACOUSTICS. 


Figure  25. — If  the  plate  be  confined  near  one  of  its  angles,  and 
the  bow  applied  to  the  middle  of  one  of  its  sides,  the  sand  will  be 
arranged  as  shown  by  the  dots  in  the  figure. 

FIG.  25. 


The  space  between  the  nodal  lines  is  just  double  the  distance  between 
the  nodal  lines  and  the  edges  of  the  plate.  The  signs  plus  and  minus 
represent  opposite  phases  of  vibration. 

FIG.  26. 


FIG.  27. 


Figure  26. — If  a  circular  plate  of  glass  be  confined  at  the  centre 

and  the  violin-bow  drawn  across  the  edge, 
the  sand  will  take  the  position  represented 
by  the  dots. 

Figure  27  represents  another  nodal 
figure  of  a  circular  plate. 

Many  hundred  forms  of  nodal  figures  have 
been  determined.  Triangular  and  polyg- 
onal plates  all  give  symmetrical  figures, 
analogous  to  those  obtained  with  square 
plates,  as  represented  by  the  following  two 
illustrations. 


ACOUSTICS. 

Figure  28  represents  a  nodal  figure  of  a  polygonal  plate. 

FIG.  28. 


319 


Figure  29  represents  nodal  figures  of  a  triangular  plate. 

FIG.  29. 


Refraction  of  Sound. 

581.   Figure  30.— Refraction  of  sound.— Although  sound 
is  reflected  by  any  surface  of   different  density  from  that  in  which  it 

FIG.  30. 


320  ACOUSTICS. 

originates,  the  sound  also  enters  the  second  medium  by  means  of  new 
vibrations,  originating  at  the  interposed  surface. 

A  cell,  S,  made  of  two  films  of  collodion,  united  at  the  edges,  and 
having  the  form  of  a  double  convex  lens,  as  shown,  will  serve  to  demon- 
strate refraction  of  sound.  The  cell  is  held  at  the  edges  by  a  frame, 
and  provided  with  an  opening,  by  which  it  can  be  filled  with  different 
gases. 

If  the  cell  be  filled  with  carbonic  acid  gas,  which  transmits  sound- 
waves with  less  velocity  than  air  (540),  the  waves  will  be  converged  by 
passing  through  the  cell. 

A  watch,  W,  held  on  the  axis  of  the  lens-shaped  cell  will  be  heard 
on  the  opposite  side  of  the  cell,  at  some  point,  as  at  the  small  end 
of  the  funnel  E,  on  the  axis.  If  the  watch  be  held  nearer  to  the  cell, 
the  ticking  will  be  heard  at  a  greater  distance  from  it ;  but  if  the  watch 
be  held  at  a  greater  distance  from  the  cell,  then  the  ticking  will  be 
heard  at  a  nearer  point.  If  the  watch,  the  lens,  or  the  ear  is  placed  out 
of  the  line  of  the  axis  of  the  cell,  the  ticking  cannot  be  heard.  Hence, 
the  refraction  takes  place  in  accordance  with  the  principles  of  refrac- 
tion of  light. 

Let  the  sound-waves  be  represented  not  by  the  lines,  but  by  the 
spaces  between  the  lines.  Then,  as  the  outer  space,  or  wave,  from  the 
watch,  comes  in  contact  with  the  cell,  that  portion  of  the  wave  falling 
on  the  cell  first  will  be  retarded  more  than  that  portion  meeting  the 
cell  later,  which,  of  course,  will  bend  the  wave  toward  the  axis,  or 
principal  perpendicular  line.  But,  in  accordance  with  this,  if  the  cell 
were  filled  with  some  medium,  as  hydrogen  gas,  which  transmits  sound 
with  greater  velocity  than  air,  then  the  waves  would  be  refracted  away 
from  the  axis  or  perpendicular  line. 

582.  The  laws  of  the  refraction  of  sound  are— 

1.  Sound-waves  passing  obliquely  into  a  medium  of  different  density, 
will  be  refracted. 

2.  If  they  travel  more  rapidly  in   the  new  medium,  they  will  be 
lent  away  from  a  perpendicular  drawn  to  the  surface  of  that  medium. 

3.  If  they  travel  less  rapidly  in  the  new  medium,  they  will  be  lent 
toward  the  perpendicular  drawn  to  the  surface  of  that  medium. 

Sounds  from  Pipes. 

583.  Sound  from  pipes. — Air  put  into  vibration  in  a  pipe,  or 
hollow  tube,  yields  a  sound.     The  air  is  the  sonorous  body ;  the  charac- 
ter of  the  sound  depending  upon  the  form  of  the  pipe,  and  the  manner 
in  which  the  vibrations  of  the  air  are  produced. 


ACOUSTICS.  321 

The  contained  air  is  thrown  into  successive  condensations  and  rare- 
factions by  introducing  a  current  of  air  through  a  suitable  mouth- 
piece.  Two  principal  forms  are  given  to  the  mouthpiece.  In  one  of 
these  the  parts  are  fixed,  and  in  the  other  there  is  a  moveable  tongue, 
called  a  reed. 

The  difference  in  the  quality  of  the  tones  produced  by  pipes  of  dif- 
ferent materials,  may  be  owing  to  a  feeble  vibration  of  the  pipes  them- 
selves. 

584.  Figure  31.— Pipes  with  fixed  mouthpieces.— These 

are  made  of  wood  or  metal,  are  rectangular  or  cylindrical,  and  are  of 
considerable  length  compared  with   their    cross-section. 
The  flute,  the  organ-pipe,  the  whistle,  or  flageolet,  etc.,  are 
examples  of  this  class  of  pipes. 

One  of  the  forms  given  to  this  class  is  represented  by 
the  figure.  H  represents  the  tube  through  which  air  is 
forced  into  it  by  the  bellows.  The  air  passes  through  a 
narrow  opening,  i,  called  the  vent.  Opposite  the  vent  is 
an  opening,  m,  in  the  side  of  the  pipe,  called  the  mouth. 
The  upper  border  of  the  mouth  is  beveled,  and  is  called 
the  upper  lip,  the  lower  border  (not  beveled)  is  called  the 
lower  lip. 

When  the  air  is  forced  through  the  vent,  i,  it  encounters 
the  edge  of  the  upper  lip,  by  which  it  is  partially  ob- 
structed, causing  a  shock,  so  that  the  air  passes  through 
the  mouth,  m,  in  an  intermitted  manner.  These  pulsa- 
tions are  transmitted  to  the  air  in  the  tube,  making  it  vi- 
brate, and  thus  producing  a  sound. 

In  order  to  have  a  pure  sound,  there  must  exist  a  cer- 
tain relation  between  the  dimensions  of  the  lips  and  the 
opening  of  the  mouth;  and  the  length  of  the  tube  must 
bear  a  certain  ratio  to  its  diameter.  Holes  or  openings  in 
the  side  of  a  wind-instrument,  as  the  flute,  flageolet,  etc., 
have  the  effect  of  virtually  varying  the  length  of  the  tube. 

585.  Reed-pipes. — In   reed-pipes  the  mouthpiece  is  provided 
with  a  vibrating  tongue,  called  a  reed.     The  reed  is  made  of  elastic 
metal,  or  wood,  and  attached  to  an  opening  in  such  a  manner  that  a 
current  of  air,  passing  into  the  opening,  causes  the  reed  to  vibrata 
This  vibration  is  propagated  to  the  surrounding  air. 

Some  of  the  reed  instruments  are  the  clarionet,  the  trumpet,  the 
bassoon,  the  accordion,  the  Jews-harp  (560),  etc. ;  the  last  being  the 
most  simple  of  this  species  of  instruments. 

21 


322 


ACOUSTICS. 


FIG.  32. 


586.  Figure  32. — Arrangement  of  reeds. — The  reeds  may 
be  so  arranged  as  to  beat  against  the  sides  of  the  opening,  or  they  may 
play  freely  through  the  opening. 

The  figure  shows  the  arrangement  of  a  reed  of  the  first  kind.  A 
piece  of  metal,  /,  shaped  somewhat  like  a  spoon,  is  fitted  to  an  elastic 
tongue,  I)  which  can  completely  close  the  opening 
shown  between  them.  A  piece  of  metal,  i,  which 
can  be  elevated  or  depressed  by  a  rod,  L,  serves  to 
shorten  or  lengthen  the  vibrating  part  of  the  reed ; 
which,  of  course,  increases  or  diminishes  the  rapidity 
of  vibration. 

When  a  current  of  air  is  forced  into  the  tube, 
TN  (the  front  of  which  is  cut  away  to  show  the 
parts  just  described),  the  reed,  I,  rapidly  vibrates, 
producing  a  succession  of  rarefactions  and  conden- 
sations in  the  air  of  the  pipe,  S,  thus  causing  it  to 
emit  sound.  The  air  entering  TN,  first  closes  the 
opening  by  pressing  the  reed  against  it;  the  reed 
then  recoils  by  the  force  of  its  elasticity,  permitting 
a  portion  of  condensed  air  to  enter  the  pipe,  when 
the  reed  is  again  pressed  against  the  opening,  and 
so  on. 

587.  The  organs  of  the  voice  a  reed  in- 
strument.— At  the  top  of  the  trachea,  or  windpipe, 
is  a  pair  of  elastic  bands,  called  the  vocal  cords, 
stretched  across  the  opening  of  the  trachea,  so  as 
nearly  to  close  it,  and  forming  a  kind  of  double  reed. 
When  the  air  is  forced  from  the  lungs  through  the  slit  between  these 
cords,  they  are  made  to  vibrate.  Their  rate  of  vibration,  within  cer- 
tain limits,  is  varied  at  will  by  changing  their  tension,  upon  which 
depends  the  pitch  of  the  voice.  The  cavities  of  the  mouth  and  nose 
act  as  resonant  tubes. 

The  various  organs  which  constitute  the  entire  vocal  apparatus  of 
man,  are  the  lungs,  the  trachea,  the  larynx,  the  pharynx,  the  mouth, 
and  the  nose,  with  their  appendages. 

A  minute  description  of  the  construction  of  these,  and  an  explana- 
tion of  the  part  that  each  performs  in  the  utterance  of  sound  and 
speech,  belong  to  the  department  of  Anatomy  and  Physiology 

Musical  Sounds. 

It  cannot  be  expected  that  more  than  a  few  leading  principles  re- 
lating to  musical  sounds,  would  be  explained  in  an  elementary  com- 


ACOUSTICS.  323 

pendium  for  schools.  Music  and  musical  instruments  are  the  subjects 
of  a  special  treatise. 

588.  Difference  bet-ween  musical  sounds  and  noises. — 

A  musical  sound  results  from  a  succession  of  atmospheric  vibrations 
of  equal  duration.  Noise  is  the  sensation  produced  by  unequal  vibra- 
tions. If  a  stone  be  thrown  into  the  middle  of  a  still  sheet  of  water,  a 
single  wave  circles  off  to  the  shore,  which  may  illustrate  the  effect  upon 
the  air  when  a  tone  is  produced.  If  several  stones  be  thrown  into  the 
water  together,  each  stone  produces  its  own  circle,  and  the  several 
circles  intersect  each  other  and  become  confused  to  the  eye.  This 
may  be  compared  to  the  effect  upon  the  air  when  a  noise  is  produced. 

589.  Qualities  of  sound.— The  ear  distinguishes  three  qualities 
of  sound :  1.  Pitch,  or  tone,  which  depends  upon  the  frequency  of  the 
vibrations.     Eapid  vibrations  yield  acute  or  high  sounds,  and  slow 
vibrations  give  low  or  grave  sounds. 

2.  The  intensity,  by  virtue  of  which  sounds  are  loud  or  soft.     Loud- 
ness  depends  upon  the  amplitude  of  the  oscillations. 

3.  Quality,  in  virtue  of  which  sounds  of  the  same  intensity  and 
pitch  are  relatively  distinguishable. 

590.  Limits  of  perceptible  sounds.— The  gravest  perceptible 
sound  is  produced  by  16  vibrations  per  second,  and  the  most  acute,  by 
48,000  vibrations  per  second.     Supposing  the  velocity  of  sound  to  be 
1,090  feet  per  second,  the  length  of  the  waves  of  the  gravest  sound 
would  be  68  feet,  and  those  of  the  most  acute,  a  little  more  than  a 
quarter  of  an  inch. 

The  limits  in  music  are  much  narrower,  especially  in  singing.  The 
lowest  sound  of  the  male  voice  being  190  vibrations  per  second ;  for 
the  female  voice,  572 ;  for  the  highest  sound  of  the  male  voice,  678 
vibrations ;  for  the  female  voice,  1,606. 

591.  Unison. — Sounds  produced  by  the  same  number  of  vibra- 
tions per  second  are  said  to  be  in  unison. 

592.  Melody. — Chord. — Harmony. — When  the  vibrations  of 
a  progressive  series  of  single  musical  sounds  bear  to  each  other  such 
simple  relations  as  are  readily  perceived,  an  agreeable  impression  is  pro- 
duced, called  melody. 

When  two  or  more  sounds,  having  to  each  other  such  simple  rela- 
tions, are  produced  simultaneously,  it  is  called  a  chord. 

A  succession  of  chords,  succeeding  each  other  in  melodious  order, 
constitutes  harmony. 


324  ACOUSTICS. 

It  is  invariably  found  that  the  sounds  caused  by  vibrations  which 
are  to  each  other  in  some  simple  numerical  proportion,  are  pleasing ; 
such  as  1  to  2,  2  to  3,  3  to  4,  etc.  The  science  of  music  does  not  admit 
of  any  proportions  except  those  which  arise  from  the  limited  combina- 
tion of  these  very  simple  numbers. 

593.  The  principal  harmonies. — The  principal  harmonies  are 
represented  in  the  following  diagrams,  the  upper  line  in  each  represent- 
ing the  acute  and  the  lower  the  grave  notes.  Those  vibrations  which 
occur  simultaneously,  and,  therefore,  increase  each  other's  power,  are 
connected  by  vertical  lines. 


Octave  BmB^MMHH     Ratio  1  to  2 

Fifth  -   m         "     2  to  3 


Fourth  "     3  to  4 

IHHMUM 

Major  Third  «     4  to  5 

fcJl^K9^K^Ml39KSHi 

. 

Minor  Third  "     5  to  6 


The  concord  1  to  2  is  most  pleasing,  every  second  vibration  of  the 
acuter  chord  coinciding  perfectly  with  each  vibration  of  the  graver ; 
it  is  called  the  octave,  as  it  comprehends  an  interval  of  eight  notes  in 
the  musical  scale.  The  concord  2  to  3  is  the  next  most  pleasing,  each 
third  vibration  of  the  acuter  corresponding  with  the  second  of  the 
graver;  it  is  called  ike  fifth,  as  it  comprehends  an  interval  of  five  notes 
from  the  fundamental  in  the  musical  scale.  The  concord  3  to  4  is 
quite  pleasing ;  it  is  called  a  fourth,  as  it  comprehends  an  interval  of 
four  notes  in  the  scale  ;  each  fourth  vibration  of  the  acuter  chord  cor- 
responds with  the  third  of  the  graver.  The  concord  4  to  5  is  pleasing  : 
it  is  called  the  major  third,  because  it  not  only  comprehends  an  interval 
of  three  notes,  but  its  ratio  4  to  5  is  greater  than  the  ratio  5  to  6,  which 
also  comprehends  an  interval  of  three  notes,  and  is  called  a  minor 


ACOUSTICS.  325 

third  ;  in  the  first  case,  five  pulsations  of  the  acuter  chord  correspond 
to  four  of  the  graver,  and  in  the  latter,  six  of  the  acuter  to  five  of  the 
graver. 


594-  T*16  most  pleasing  harmonies.  —  The  combination  of 
two  notes  is  the  more  pleasing  to  the  ear,  the  smaller  the  two  numbers 
which  express  the  ratio  of  their  vibrations. 

595.  The   limit  of  harmonies.  —  The  limit  beyond  which  a 
musical  ear,  and  the  mind  generally,  will  not  tolerate  the  combination 
of  two  sounds,  is  that  expressed  by  5  to  6,  or  that  of  minor  third. 

596.  Musical  scale.—  Gamut.  —  The  tones  forming  a  melodi- 
ous series  between  any  two  adjacent  sounds  which  are  as   1   to  2, 
are  called  the  musical  scale  or  gamut. 

The  sounds  which  compose  the  musical  scale  or  gamut,  are  the 
alphabet  of  music.  To  find  the  relation  which  exists  between  the 
fundamental  note  (C)  and  the  other  notes,  the  sonometer  is  em- 
ployed (573). 

The  names  of  the  sounds  composing  the  scale  are,  in  English,  C,  D, 
E,  P,  G,  A,  B.  In  French  and  Italian,  do,  re,  mi,  fa,  sol,  la,  si. 

By  means  of  the  sonometer,  it  is  found  that  the  length  of  the  cord 
corresponding  to  each  note  is  represented  by  the  following  fractions  : 
Notes  .........................  CDEFGABC' 

Relative  length  of  cord  ..........   IftlllAl 

597.  Formation  of  the  musical  scale.  —  It  has  been  shown 
that  the  number  of  vibrations  is  in  the  inverse  ratio  of  the  length  of 
the  string  (572).     Hence,  the  relative  number  of  vibrations,  corre- 
sponding to  each  note  in  the  same  time,  will  be  expressed  by  inverting 
the  fractions  of  the  preceding  table. 

Representing,  therefore,  the  number  of  vibrations  corresponding  to 
the  fundamental  note  C,  by  1,  we  have  : 

Notes  .........................  C    D   E    F   G    A    B     C' 

Relative  number  of  vibrations  ....  1     £    £    -J    f     J    *£-    2 

To  avoid  fractions,  whole  numbers  bearing  the  same  ratio  may  be 
substituted,  thus: 

CDEFGABC' 
24    27    30   32   36    40    45   48 

Absolute  number  of  vibrations  corresponding  to  each 
note.  —  The  notes  of  the  scale  whose  gamut  corresponds  to  the  gravest 
sound  of  the  bass  are  indicated  by  1.  To  notes  of  gamuts,  more  ele- 


326 


ACOUSTICS. 


vated,  are  affixed  the  indices  2,  3,  etc. ;  to  graver  notes  are  affixed  the 
indices  — 1,  — 2,  etc.  The  number  of  simple  vibrations  corresponding 
to  the  note  0,  is  128  per  second.  Hence,  by  multiplying  this  number 
by  the  several  fractions  (597),  we  have : 

Notes C       D       E        F        Gr       A        B 

Absolute  number  of  )      m    144    16Q    mf    m    ^        M() 
simple  vibrations    ) 

The  absolute  number  of  vibrations  for  superior  gamut  is  obtained 
by  multiplying  the  numbers  in  this  table  successively  by  2,  by  3,  etc. 

The  following  table  indicates  the  length  of  the  waves  corresponding 
to  the  0  of  successive  scales : 


LENGTH  OF  WAVES 
IN  FEET. 

NUMBER   OF   VIBRATIONS 
IN  A  SECOND. 

C—  3 

70 

16 

C—  2 

35 

32 

0—1 

17.5 

64 

01 

8.73 

128 

02 

4.375 

256 

03 

2.187 

512 

04 

1.093 

1024 

It  will  be  noticed  (597)  that  the  interval  C — C',  as  indicated  by  the 
figures,  bears  the  ratio  of  1  to  2,  and  is  called  an  octave. 

The  interval  C — Gr  is  called  a  fifth,  comprehending  five  notes,  the 
ratio  being  2  to  3. 

The  interval  C — F,  and  also  E — A,  is  called  a  fourth,  comprehend- 
ing four  notes,  the  ratio  in  each  case  being  3  to  4. 

The  interval  C — E  and  F — A  is  called  the  major  third,  the  ratio 
being  4  to  5,  and  the  interval  three  notes. 

The  interval  E — Gr  and  A — 0'  is  called  the  minor  third,  the  inter- 
val being  three,  and  the  ratio  5  to  6,  which  is  less  than  the  ratio  4  to  5 ; 
hence  the  -name,  minor  third. 


MAGNETISM. 


327 


CHAPTEE    XIII. 

(CHART  NO.  7.) 

MAGNETISM. 
GENERAL    PROPERTIES    OF    MAGNETS. 

598.  Definitions. — Magnetism,  as  a  science,  is  that  branch  of 
Physics  which  treats  of  the  properties  of  magnets,  and  of  their  action 
upon  each  other,  and  upon  other  bodies. 

The  real  nature  of  the  magnetic  force  is  unknown ;  but  the  analogies 
oifered  by  electro-magnetism  and  magneto-electricity  indicate,  that  it  is 
one  mode  of  electrical  excitement. 

599.  Lodestone,  or  natural  magnets.— Lodestone  (to  lead 
and  stone)  is  an  ore  of  iron,  found  in  a  natural  state  in  many  parts  of 
the  earth,  possessing  the  power  of  attracting  iron  and  a  few  other  sub- 
stances.    This  power  is  called  magnetism,  from  the  name  of  the  ancient 
city  Magnesia,  near  which  this  ore  was  first  found.     It  is  a  compound 
of  one  equivalent  of  peroxyd  of  iron  with  one  of  protoxyd. 

600.  Figure  33.— Magnetic  manifestations  of  lodestone. 

— If  a  piece  of  this  ore  be  dipped  in  iron  filings  they  will  collect  and 
cling  together  at  two  opposite  extremities,  as  rep-  FIG.  33. 

resented  in  the  figure.  The  magnetic  property, 
whatever  it  may  be,  seems,  therefore,  to  be  col- 
lected, and  to  act  with  the  greatest  energy,  at 
two  opposite  extremities.  These  are  termed  poles 
(512). 

If  a  piece  of  the  ore,  as  shown,  be  laid  down  on 
a  piece  of  board,  and  the  board  floated  on  water, 
the  lodestone  will  invariably  arrange  itself  so  that 
the  same  pole  will  be  directed  to  the  north  and 
the  other  to  the  south.  Hence  the  pole  which 
turns  to  the  north  is  called  the  North  pole  and 
the  other  the  South  pole,  as  indicated  by  the  let- 
ters in  the  figure.  Pieces  of  this  ore  are  called 
natural  magnets. 


328 


MAGNETISM. 


601.  Figure  34.— The  armature.— The  effective  power  of  the 
lodestone  is  improved  by  meaDS  of  what  is  termed  an  armature  ;  which 

FIG.  34. 


consists  of  pieces  of  soft  iron,  AE  and  TH,  applied  to  the  opposite  polar 
surfaces,  S  and  N,  of  the  lodestone,  M.  The  attractive  force  is  thus 
transmitted  to  the  artificial  poles  of  iron,  E  and  H. 

602.  Figure  35.— A  fully-mounted  lodestone  magnet. 

— This  consists  of  the  armatures  secured  to  the  lodestone,  M,  by  brass 
FIG.  35.  binders,  A  and  E;   a  ring,  H,   for 

suspending  the  apparatus  ;  a  keeper, 
K,  which  is  a  soft  piece  of  iron,  con- 
necting the  poles,  S  and  N,  as  shown. 
This  keeper  is  found  to  preserve 
and  increase  the  attractive  force  of 
the  poles,  especially  if  the  whole  be 
suspended  by  the  ring,  and  weights 
be  attached  to  the  hook  of  the 
keeper. 

60S.  Artificial  magnets  are 

bars  of  iron,  steel,  or  tempered  steel, 
to  which  the  property  of  the  natural 
magnet  has  been  imparted. 

Artificial  magnets  are  either  per- 
manent or  temporary.  Permanent 
magnets  are  made  of  tempered  or 

hardened  steel ;  temporary  magnets  are  made  of  untempered  steel  and 

of  soft  iron. 


MAGNETISM. 


329 


Artificial  magnets  are  more  powerful  and  useful  than  the  lodestone 
or  natural  magnet,  and  possess  properties  identical  with  it.  Tem- 
porary magnets  do  not  retain  their  magnetism  after  the  exciting  cause 
is  removed. 


FIG 


604-  Figure  36.— Method  of  making  an  artificial  mag- 
net with  lodestone. — ME  represents  a  part  of  the  mounted  natural 
magnet  (602),  s  and  K  being  its  poles. 

To  impart  the  magnetism  to  the  tempered  bar 
of  steel,  A,  and  render  it  a  permanent  magnet, 
place  it  upon  the  poles,  as  shown  in  the  figure, 
and  slide  it  lengthwise  back  and  forth  a  number 
of  times,  but  not  so  far  as  to  pass  either  extrem- 
ity of  the  bar  beyond  either  pole ;  finally  bring 
the  bar  at  rest,  with  its  ends  at  an  equal  dis- 
tance from  the  poles,  and  then  lift  it  perpen- 
dicularly from  the  natural  magnet.  The  bar 
will  now  manifest  magnetic  attraction  and  po- 
larity. 

The  artificial  magnet  will  have  opposite  po- 
larity to  the  original  magnet ;  that  is,  the  south 
pole  of  the  artificial  magnet  will  have  been 
developed  by  or  at  the  north  pole  of  the  natural  magnet,  and  vice 
versa. 

605.  Figure  37.— Distribution  of  force  in  magnets.— The 

magnetic  force  is  not  equally  distributed  in  all  parts  of  a  magnet. 
The  attraction  is  strongest  at  the  ends,  and  dimin-  FIG.  37. 

ishes  toward  the  centre,  where  it  is  neutral.  The 
ends,  where  the  attraction  is  greatest,  are  called 
poles ;  the  central  part,  where  the  attraction  is 
nothing,  is  called  the  equator,  or  the  neutral  line. 

The  positions  of  the  poles  and  equator  are  shown 
by  the  figure,  in  which  the  radiating  lines  represent 
the  attractive  force.  If  the  magnet  be  rolled  in 
iron  filings,  the  particles  of  iron  will  be  attracted  to, 
and  held  in  the  position  of,  these  radiating  lines. 

Every  magnet  has  at  least  two  poles,  and  one 
neutral  point. 

The  poles  are  distinguished  by  North  and  South 
(N  and  S),  Austral  or  Boreal   (A  and  B),  or  by  the 
signs  plus  (  +  )  and  minus  (•—)•    These  signs  refer  to  the  earth's  attrac- 
tion and  antagonism  between  poles  of  unlike  names. 


330 


MAGNETISM. 


Sometimes,  owing  to  inequality  of  temper  in  the  steel,  artificial  mag- 
nets have  minor  poles,  situated  between  the  principal  ones,  called 
secondary  poles. 

606.  The   law  of  distribution  of  attraction.  —  The  law 

regulating  the  distribution  of  magnetic  force  in  a  bar  is,  that  the  force 
is  nearly  as  the  square  of  the  distance  of  any  given  point  from  the 
magnetic  equator. 

607.  The  force  of  magnetic  attraction  at  different  dis- 
tances.— Magnets  attract  at  all  distances,  but  their  power  decreases  as 
the  square  of  the  distance  from  their  poles  increases,  being  in  accordance 
with  the  common  law,  that  regulates  all  forces  which  act  from  a  centre, 
as  that  of  heat,  light,  gravitation,  etc. 

The  most  powerful  magnets  sustain  about  thirty  times  their  own 
weight. 

608.  Effect  of  heat  on  magnets. — The  power  of  magnets  is 
diminished  by  heat;  but  if  they  be  heated  only  to  redness,  the  power 
returns  on  cooling.     TFMe-heat  wholly  destroys  their  magnetic  force. 
Their  power  is  increased  at  low  temperatures. 


FIG.  38. 


609.  Figure  38.— Various  forms  of  mag- 
nets.—The  bar  magnet  is  a  simple  straight  bar  of 
steel.  The  horse-shoe  magnet  is  the  bar  magnet  bent 
in  the  form  of  a  horse-shoe. 

The  most  powerful  attraction  takes  place  when 
both  poles  can  be  applied  to  the  surface  of  a  piece  of 
iron  at  once ;  hence,  to  facilitate  such  an  application, 
many  magnets  are  made  in  the  horse-shoe  form. 

A  compound  magnet  consists  of  several  magnets 
bound  together.  The  figure  represents  a  compound 
magnet  made  of  several  plain  bar  magnets. 

The  dimensions  well  adapted  to  magnetic  bars, 
straight  or  curved,  are  such  as  to  give  the  breadth 
about  Tij-  or  -^  of  the  length,  and  the  thickness  not 
more  than  half  the  breadth. 


610.  Figure  39. — Compound  horse-shoe  magnet. — This 
consists  of  several  horse-shoe  magnets  bound  together,  with  their  simi- 
lar poles  in  contact,  by  means  of  a  clasp  at  the  middle  or  neutral 
point,  and  with  rivets  or  screws  at  the  ends,  as  shown. 

To  preserve  the  magnetism,  the  poles  are  kept  united,  when  the  magnet 


MAGNETISM.  331 

is  not  in  use,  by  means  of  a  soft  bar  of  FlG-  39- 

iron,  called  the  keeper.  Such  magnets  are 
called  magnetic  batteries,  and  are  often  em- 
ployed for  charging  other  magnets. 

Large  magnets  are  not  as  powerful,  in 
proportion  to  their  weight,  as  small  ones. 

There  is  found  a  limit  beyond  which 
there  is  no  advantage  in  extending  these 
batteries. 

Charging  Magnets. 

611.  Method  of  charging  mag- 
nets.— Artificial   magnets  are  produced 
not  only  by  touch  or  friction  from  other 
magnets  ;  but  by  induction  ;  and  by  elec- 
trical currents. 

The  method  by  touch  is  accomplished 
by  various  modes  of  manipulation,  of  which  only  two  or  three  will  be 
described. 

612.  Figure  40.— Method  of  charging  horse-shoe  mag- 
nets.— Let  L  be  the  bar  to  be  charged  or  magnetised.     Having  secured 
it  on  a  board,  and  united  its  extremities  with  a  bar  of  soft  iron,  K, 
place  the  compound  magnet,  F,  on  one  arm  of  the  bar,  as  shown,  and 

FIG.  40. 


glide  it  around  on  the  two  arms  several  times ;  and  having  brought  it 
to  a  state  of  rest  at  the  neutral  point,  L,  remove  it.  Then  turn  the 
bar  over  and  repeat  the  operation  on  the  other  side,  always  observing 


332  MAGNETISM. 

to  keep  the  unlike  poles  of  the  two  magnets  in  contact  with  each  other ; 
that  is,  so  that  the  N  pole  of  the  magnetizing  magnet  will  be  toward 
the  S  pole  of  the  magnet  which  is  being  charged,  as  indicated  by  the 
several  letters. 

613.  Figure  41. — Method  of  magnetizing  straight 
bars. — Straight  bars,  and  needles  for  compasses,  are  usually  magnet- 
ized by  rubbing  them  with  other  bar-magnets.  There  are  three  ways 
of  doing  this,  known  as  the  methods  by  single  touch,  by  separate  touch, 
and  by  double  touch. 

By  single  touch,  the  bar  is  magnetized  by  simply  passing  one  end  of 
a  powerful  bar-magnet  several  times  over  it. 

By  separate  touch,  the  bar  is  magnetized  by  being  rubbed  in  one 
direction  with  one  pole  of  the  magnet,  and  in  the  opposite  direction 
with  the  opposite  pole. 

To  magnetize  by  double  touch,  two  or  four  magnets  are  employed, 

FIG.  41. 


which  may  be  simple  or  compound.     In  the  drawing,  four  simple  mag- 
nets, and  the  method  of  using  them,  are  represented. 

Let  H  represent  the  bar  to  be  magnetized.  First  place  it  upon  the 
opposite  poles  of  two  magnets  (only  one  end  of  each  of  which  is 
shown),  then  take  two  other  magnets,  E  and  F,  and,  having  placed 
them  as  represented  (that  is,  with  their  similar  poles  reversed),  simul- 
taneously draw  them  from  the  centre  to  the  extremities  of  the  bar,  as 
indicated  by  the  two  arrows;  then  lift  them  up  a  foot  or  so,  and  again 
place  them  in  the  same  position,  repeating  the  process  many  times  on 
both  sides  of  the  bar. 


MAGNETISM. 


333 


The  N  pole  of  the  new  magnet  will  be  formed  between  the  south 
poles  of  the  magnets,  and  its  S  pole  between  the  north  poles,  as  indi- 
cated by  the  letters. 

614-  Figure  42.— Both   poles   must  coexist   in   every 
magnet. — If  a  magnet  be  broken  at  the  neutral  point,  as  shown,  each 
half  will  become  a  complete   magnet  of  diminished          FIG.  42. 
force,  having  two  opposite  poles,  like  the  original.    The 
poles,  formed  at  the  broken  ends,  will  be  opposite  in 
character  to  those  of  the  corresponding  extremities  of 
the  original  magnet. 

If  these  fragments  be  again  and  again  broken,  to  the 
extreme  degree  of  mechanical  fineness,  each  particle, 
however  small,  will  be  a  perfect  magnet. 


61 5.  Magnetic  and  magnetized  bodies. — A 

magnetic  body  is  one  which  contains  the  two  magnetic 
fluids  or  forces,  but  in  a  state  of  equilibrium;  thus, 
iron,  steel,  nickel,  and  cobalt  are  such  bodies. 

Magnetized  bodies  contain  the  two  fluids,  but  in  a 
state  of  separation,  each  producing  an  opposite  effect, 
whilst  in  the  magnetic  bodies  the  fluids  are  combined 
and  produce  no  effect. 

Induction. 

616.  Figure  43. — Induction.— Magnetism  by  contact.— 

Let  M  represent  a  bar-magnet,  with  its  poles  arranged  as  shown  by  the 

letters  N  and  S.    If  a  magnet-  FlG  43 

ic  body,  as   an   iron   key,  be 

placed  in  contact  with  one  of 

the  poles  of  the  magnet,  it  will 

be  magnetized,  and  adhere  to 

the  magnet. 

If  a  second  key  be  brought 
in  contact  with  the  first,  it 
also  will  be  magnetized  and 
adhere  as  shown,  and  so  on. 
If  iron  keys,  or  other  magnet- 
ic bodies,  be  brought  in  con- 
tact with  the  other  pole  of  the 
magnet,  they  will  be  similarly 
magnetized;  the  only  differ- 
ence being,  that  their  respec- 
tive poles  will  be  reversed. 


334 


MAGNETISM. 


In  both  cases  the  magnetized  bodies  in  contact  with  the  magnet 
have  their  poles  reversed ;  or,  generally,  the  adjacent  extremities  of  all 
the  bodies  (including  the  magnet)  have  opposite  polarity,  as  indicated 
by  the  letters. 

If  one  of  these  magnetized  bodies  be  reversed,  or  turned  end  for  end, 
its  polarity  will  also  be  reversed ;  that  is,  the  end  that  was  the  N  pole 
becomes  the  S  pole,  and  vice  versa. 

This  action  of  a  magnet  upon  magnetic  bodies  is  called  induction. 

Some  substances  besides  the  so-called  magnetic  metals,  become 
slightly  magnetic.  Some  minerals  become  magnetic  by  heating ;  and 
the  alloy,  brass,  by  hammering.  The  pure  earths,  and  even  silica,  are 
found  to  have  the  same  property.  See  Diamagnetism,  783. 

617.  Figure  44.— Magnetic  induction  illustrated  by  a 
series  of  rings. — If  an  iron  ring  be  placed  in  contact  with  a  magnet, 
it  will,  by  induction,  become  itself  a  magnet.  If  a  second  ring  be  pre- 
sented to  the  first,  it  will  in  like  manner  become  a  magnet,  and  so  on, 
with  a  considerable  number  of  them,  as  represented.  If  the  bar  be 
removed,  the  rings  lose  their  magnetism,  cease  to  adhere,  and  the  chain 
falls  to  pieces. 


FIG.  44. 


FIG.  45. 


618.  Figure  45.— Arrangement  of  poles  by  induction 
in  a  star-shaped  body. — If  a  piece  of  sheet-iron  be  cut  in  the 
form  of  a  star,  and  one  end  of  a  bar-magnet  be  placed  on  its  centre,  as 
represented  in  the  figure,  the  central  part  will  have  the  opposite  polari- 
ty of  the  end  of  the  magnet  in  contact  with  it,  and  the  points  will  have 
the  opposite  polarity  of  the  centre,  as  shown  by  the  letters. 


MAGNETISM. 


335 


FIG.  46. 


619.  Figure  46.— Production  of  two  sets  of  poles  in  one 
bar  by  induction.— If  a  bar-magnet,  shown  by  the  upright  part  of 
the  figure,  has  one  of  its  poles 

brought  in  contact  with  the  mid- 
dle portion  of  a  bar  of  iron,  there 
will  be  developed  at  the  point  of 
contact  a  polarity  opposite  to  that 
of  the  contact  end  of  the  magnet, 
which  will  cause  the  extremities 
of  the  magnetized  bar  to  have  like 
poles  with  each  other,  but  oppo- 
site to  that  of  the  centre,  as  shown 
by  the  letters. 

620.  Figure  47.— Induc- 
tion -without  contact. — Every 
magnet  is  surrounded  by  a  sphere 
of  magnetic  influence,  called  its 
magnetic  atmosphere.    Magnetiza- 
ble or    magnetic    bodies  within 

this  influence  become,  without  contact,  more  or  less  magnetized ;  the 
parts  contiguous  to  the  magnet-pole  having  opposite,  and  those  remote 


FIG.  47. 


from  it  a  similar  polarity,  as  shown  by  the  letters  in  the  figure ;  M  rep- 
resenting the  magnet,  and  the  other  parts  of  the  drawing,  small  bars 
of  iron. 


336 


MAGNETISM. 


621.  Figure  48. — Magnets  do  not  part  with  their  own 
power. — Magnets  do  not  part  with  or  lose  any  of  their  own  magnetic 
force  by  magnetizing  other  bodies.  They  simply  act  to  develop  or 
bring  into  action  a  power  which  already  resides  in  the  other  bodies, 
but  in  a  state  of  equilibrium.  To  prove  this,  let  F  be  a  bar-magnet, 
sustaining  a  small  bar  of  iron,  H,  to  which  is  attached  a  scale-pan. 
Place  in  the  scale-pan  just  sufficient  weight  to  cause  the  bar,  H,  to  be 
severed  from  the  magnet.  Remove  the  weights  from  the  pan,  suspend 
it  again  to  the  magnet,  and  place  the  bar  of  iron,  E,  on  the  top  of  the 
magnet,  which,  of  course,  will  be  magnetized  by  the  contact.  While 
the  bar,  E,  remains  in  this  position,  more  weight  than  before  is  required 
to  separate  the  small  bar,  H,  from  the  magnet. 

This  not  only  shows  that  the  magnet  has  lost  none  of  its  own  force, 
but  it  has  developed  a  force  in  the  bar,  E,  which,  acting  in  conjunc- 
tion with  its  own,  enables  it  to  sustain  a  greater  weight  than  it  pre- 
viously did. 


FIG.  48. 


FIG.  49. 


622.  Figure  49.— Unlike  poles  neutralize  each  other.— 

A  combination  of  two  or  more  magnets,  with  their  like  poles  in  con- 
tact, exerts  a  greater  force  than  one  alone,  or  even  greater  than  the  sum 
of  their  forces,  applied  separately.  But  if  two  equal  magnets  be  com- 
bined, with  their  unlike  poles  in  contact,  they  mutually  destroy  each 
other. 


MAGNETISM. 


337 


If  an  iron  key  be  suspended  to  the  magnet,  E,  and  another  equal 
magnet,  F,  is  brought  in  contact  with  E,  and  slid  along,  as  represented 
in  the  figure,  the  key  will  not  fall  off  if  the 
like  poles  of  the  two  magnets  are  in  contact, 
as  shown.  But  if  the  magnet,  F,  be  reversed, 
end  for  end,  the  key  will  drop  when  the  ends 
of  the  magnets  are  brought  nearly  even  with 
each  other. 

623 .  Figure    50.  —  Neutralization 
shown  by  the  Y-magnet. — Place  on  the 
arms,  H  and  K,  of  the  Y-shaped  piece  of  soft 
iron,  the  two  magnets,  E  and  F,  with  their 
like  poles  turned  in  the  same  direction,  and 
the  lower  end  of  the  Y-shaped  iron  will  at- 
tract and  hold  the  key.     If  one  of  the  mag- 
nets (say  F)  be  removed,  the   key  will  still 
remain  suspended ;  but  if  F  be  again  applied, 
with  its  poles  reversed,  as  shown  in  the  draw- 
ing, the  key  will  instantly  fall ;  because  of  the 
mutual  neutralizing  effect  of  like  poles  at  the 

bottom  of  the  Y-shaped  iron,  as  indicated  by  the  several  letters. 

624.  Figure  51.— The  inductive  power  of  the  earth's 
magnetism.— Bars  of  iron  and  steel,  by  standing  a  long  time  in  a  ver- 
tical position,  will  acquire  polarity  (646) ;  which  FIG.  51. 

is  caused  by  the  inductive  power  of  the  earth's 
magnetism.  By  testing  such  bars  with  a  small 
needle,  as  shown  in  the  figure,  it  is  found  that 
the  end  toward  the  earth  is  always  austral,  and 
the  opposite  end  boreal,  while  the  central  portion 
is  neutral. 

If  the  experiment  were  made  south  of  the 
equator  of  the  earth,  the  polarity  of  the  bar 
would  be  reversed. 

Ordinary  crow-bars,  which  have  been  kept 
standing  in  one  place  for  years  (when  not  in  use), 
are  found  more  or  less  magnetized. 

Globes  of  iron,  like  bomb-shells,  a  foot  or  more 
in  diameter,  become  miniature  copies  of  the 
earth,  as  regards  magnetism,  by  virtue  of  the 
inductive  force  of  the  earth's  magnetism. 


338 


MAGNETISM. 


Hypotheses  and  Laws  of  Magnetism. 

625.  Figure  52.— Hypothesis  of  two  magnetic  fluids. — 
The  action  of  the  two  poles  of  a  magnet  upon  a  piece  of  soft  iron  is 

52  the  same  5  but  the  action  of  the  two  poles  is 

not  the  same  upon  another  magnet.  If  a  small 
magnet,  like  the  needle  shown  in  Fig.  57 
(635),  be  balanced  on  a  pivot,  or  suspended 
at  the  neutral  point  by  a  small  string,  and 
the  N  pole  of  a  magnet  be  brought  near  to 
its  S  pole,  it  will  be  attracted  by  the  magnet. 
But  if  the  S  pole  of  the  magnet  be  brought 
near  to  the  S  pole  of  the  needle,  it  will  be  not 
only  not  attracted,  but  it  will  be  repelled  by 
the  magnet,  and  with  a  force  equal  to  the 
attraction  in  the  other  case. 

To  explain  these  phenomena,  it  is  sup- 
posed there  are  two  magnetic  fluids,  or  two 
kinds  of  subtle  matter,  surrounding  the  mole- 
cules of  the  magnet,  each  fluid  repelling  its 
own  kind,  and  attracting  the  other. 
According  to  this  theory,  a  body  is  magnetized  when  these  fluids  are 
separated,  and  driven  to  its  opposite  extremities.  Hence,  the  poles 
which  contain  the  same  kind  of  fluid  repel  each  other,  and  those  which 
contain  opposite  kinds  attract  each  other.  The  attraction  and  repul- 
sion are  mutual. 

Every  magnet,  in  this  view,  must  be  regarded  as  an  assemblage  of 
numberless  small  magnets,  every  molecule  of  steel  having  its  own 
poles  antagonistic  to  those  of  the  next  contiguous  particle. 

In  the  figure,  the  small  parallelograms  represent  the  particles  of  the 
magnet,  the  N  poles  pointing  in  one  direction,  and  the  S  poles  in  the 
opposite  direction.  These  opposing  forces,  therefore,  constantly  in- 
crease from  the  central  or  neutral  point,  where  they  are  in  equilibrium, 
to  the  ends,  where  they  are  greatest. 

626.  Laws  of  attraction  and  repulsion. — The  laws  of  mag- 
netic attraction  and  repulsion  are — 

1.  Magnetic  poles  of  contrary  names  attract,  and  those  of  the  same 
name  repel  each  other. 

2.  The  forces  of  attraction  and  repulsion  both  vary  inversely  as  the 
square  of  the  distance  between  the  attracting  and  repelling  poles. 


627.  The  coercitive  force. — The  resistance  which  bodies  show 
to  the  induction  of  magnetism  is  called  the  coercitive  force. 


MAGNETISM. 


339 


The  two  fluids  are  more  easily  separated  in  some  bodies  than  others. 
In  soft  iron  they  yield  with  less  resistance  than  in  any  other  substance ; 
but  in  hardened  steel  a  powerful  magnetic  force  is  required  to  induce 
any  permanent  magnetism.  The  harder  the  steel  the  more  difficult  it 
becomes  to  separate  the  two  fluids. 

Soft  iron  parts  with  its  induced  magnetism  as  readily  as  it  receives 
it.  The  reverse  is  the  case  with  hardened  steel ;  it  takes  time  and  force 
to  render  it  a  magnet,  but  it  retains  its  magnetism  for  a  long  time. 

The  force  which  resists  the  separation  of  the  two  fluids  acts,  after 
their  separation,  to  prevent  their  reunion. 


FIG.  53. 


Magnetic   Curves. 

628.  Figure  53.— Magnetic  curves  rendered  apparent 
to  the  eye. — If  a  piece  of  paper  be  stretched  on  a  frame,  and  placed 
over  a  powerful  bar,  SN,  the  magnetic  attraction  and  repulsion  will  be 
exerted  through  the  paper ; 

which  may  be  shown  by 
projecting  on  the  paper, 
through  a  lawn  sieve,  some 
fine  iron-dust  or  filings.  The 
particles  will  arrange  them- 
selves in  a  series  of  curved 
lines  of  magnetic  force,  pro- 
ceeding from  homologous 
or  similar  points  on  each 
side  of  the  middle  of  the  bar,  some  uniting  about  the  magnetic  centre, 
others  standing  oat  at  the  extremities,  as  if  repelled  from  the  poles,  N 
and  S,  and  tending  to  turn  at  considerable  distances  into  other  curved 
lines  of  force,  to  unite  their  branches  between  the  opposite  poles. 

629.  Figure  54. — Magnetic  curves  with  two  magnets 
and  unlike  poles. — Place  the  stretched  paper  over  dissimilar  poles, 
SN,  of  two  powerful  bar-mag- 
nets, placed  about  two   inches 

apart,  and  project  over  them 
the  fine  iron  filings  as  before. 
Magnetic  lines  of  force,  both 
straight  and  curved,  and  pro- 
ceeding from  similar  points  of 
each  bar,  will  be  apparent,  unit- 
ing the  two  poles  by  chains  of 
reciprocal  attraction. 


FIG.  54 


340  MAGNETISM. 

630.  Figure  55.— Magnetic  curves  with  two  magnets 
and  similar  poles. — Reverse  the  position  of  one  of  the  magnets  in 
the  last  experiment,  so  as  to  oppose  two  similar  poles,  1STN,  and  the 

FlG  55  lines  of  force  will  then  appear 

to  be  conflicting  lines.  The 
repulsive  forces  will  cause  a 
transverse  straight  line  to  ap- 
pear upon  the  space  between 
the  poles.  At  this  line,  the 
opposed  forces  are  struggling 
with  each  other,  being  exerted 
in  repulsive  directions  from  the 
opposed  poles. 
These  phenomena  afford  satisfactory  visual  evidence  of  the  existence 

of  two  distinct  forces,  of  their  reciprocal  attractions  and  repulsions, 

and  their  mutual  neutralization. 

631.  Magnetic    attraction    not    intercepted. —Magnetic 

attraction  acts  through  glass,  paper,  and  solid  and  liquid  substances 
generally,  which  are  not  capable  of  acquiring  magnetic  influence  in 
the  ordinary  manner.  Magnets  manifest  the  same  phenomena  in  water 
and  in  a  vacuum  as  in  air. 

632.  Preservation  of  magnets. — Magnets,  if  abandoned  to 
themselves,  would  in  time  lose  much  of  their  power ;  hence  it  is  that 
the  armatures  are  employed.    The  armature  is  a  piece  of  soft  iron  placed 
in  contact  with  the  poles  of  a  magnet. 

The  poles  acting  by  induction  upon  the  armature,  develop  its 
polarity,  and  its  two  poles  acting  on  the  two  poles  of  the  magnet,  pre- 
vent the  recomposition  of  the  two  fluids,  and  thus  preserve  its  magnet- 
ism. The  armature  is  often  called  the  keeper. 


TEKRESTKI AL    MAGNETISM 


633.  The  earth  as  a  magnet. — The  earth  may  be  considered  a 
huge  magnet,  acting  upon  magnetic  needles  in  the  same  way  that  mag- 
netized bars  do.  Its  magnetic  poles  are  near  the  geographic  poles,  and 
its  neutral  line  coincides  nearly  with  the  equator. 

The  fluid  which  is  assumed  to  predominate  at  the  north  pole  of  the 
earth  is  called  the  boreal  fluid  ;  that  which  is  supposed  to  predominate 
at  the  south  pole  is  called  the  austral  fluid. 

As  dissimilar  poles  attract  and  similar  ones  repel,  the  pole  of  a 
balanced  magnetic  needle  which  turns  toward  the  north  must  contain 


MAGNETISM. 


341 


the  austral  fluid,  and  the  one 
which  turns  toward  the  south 
must  contain  the  boreal  fluid. 


FIG.  56. 


634-  Figure  56.  The  as- 
tatic needle  is  an  instrument 
in  which  the  directive  tendency 
of  the  earth's  magnetism  is  neu- 
tralized, by  placing  two  equal 
needles,  NS,  parallel  one  above 
the  other,  with  their  unlike  poles 
opposed  to  each  other.  This  sys- 
tem is  suspended  from  a  suitable 
support  by  a  hair  or  fibre  of  raw  silk,  H,  and  is  a  sensitive  test  for 
feeble  magnetic  currents. 

635.  Figure  57.— Magnetic  needle. — The  drawing  represents 
a  simple  magnetic  needle,  being  nothing  more  than  a  piece  of  hard- 
ened steel,  tapered  from  the  pIG 

middle  to  the  extremities, 
thoroughly  magnetized,  and 
accurately  balanced  on  a  piv- 
ot, so  it  shall  be  free  to  turn 
in  all  horizontal  directions, 
as  indicated  by  the  arrows. 

636.  Directive  force 
of  magnets. — By   the  di- 
rective force  of  magnets,  is 
meant,  the  tendency   which 
they  have  to  arrange  them- 
selves in  such  a  manner  that  their  like  poles  will  be  reverse  to  each 
other,  which  is  in  obedience  to  that  fundamental  law  of  magnetic 
attraction,  which  causes  like  poles  to  repel  and  unlike  poles  to  attract 
each  other. 

The  balanced  magnetic  needle  assumes  its  position  in  obedience  to 
the  same  law,  and  comes  to  rest  with  its  austral  or  N  pole  toward  the 
north  pole  of  the  earth,  and  its  boreal  or  S  pole  toward  the  south  pole 
of  the  earth.  Hence,  what  we  generally  call  the  north  pole  of  a  needle 
is  really  its  south  pole,  and  its  south  pole  is  its  north  pole. 

For  simplicity,  the  mariner's  compass  and  other  needles  are  simply 
marked  N  on  that  point  which  turns  to  the  north. 

All  bar-magnets,  free  to  move  in  a  horizontal  plane,  arrange  them- 


342  MAGNETISM. 

selves  in  this  manner  in  all  parts  of  the  earth.  Hence,  the  earth  is 
regarded  as  an  immense  magnet,  controlling  the  position  of  small 
magnets.  The  directive  power  of  the  earth  is  accounted  for  in  another 
way,  which  will  be  explained  hereafter. 

The  directive  force  simply  rotates  the  magnet  or  nee- 
dle.— If  the  needle  be  attached  to  a  piece  of  cork,  and  placed  in  a  dish 
of  water,  it  will  turn  and  come  to  rest  in  the  same  general  direction  as 
though  it  were  balanced  on  a  pivot.  Though  the  needle  is  now  free  to 
move,  it  does  not  advance,  either  toward  the  north  or  south.  Hence, 
it  is  inferred  that  the  force  exerted  upon  the  needle  is  simply  a  direc- 
tive force. 

637.  Magnetic  meridian. — When  a  balanced  magnetic  needle 
comes  to  a  state  of  rest,  it  lies  in  the  direction  of  magnetic  north  and 
south.     The  imaginary  plane  passing  through  the  needle  and  the  centre 
of  the  earth,  is  called  the  plane  of  the  magnetic  meridian,  or  the  mag- 
netic meridian.    A  plane  passing  through  the  place  and  the  axis  of  the 
earth,  is  called  the  plane  of  the  true  meridian,  or  the  true  meridian. 

Variations  of  the  Needle. 

638.  Declination  of  the  needle. — The  magnetic  meridian  and 
the  true  meridian,  in  general,  do  not  coincide  with  each  other.     The 
angle  which  the  magnetic  meridian,  at  any  place,  makes  with  the  true 
meridian,  is  called  the  declination  of  the  needle.    In  other  words,  the 
declination  of  the  needle  is  its  deviation  from  true  north  and  south. 
This  is  different  at  different  places,  and  at  the  same  place  at  different 
times. 

If  the  north  end  of  the  needle  rests  on  the  east  side  of  the  true  merid- 
ian, that  is,  points  east  of  true  north,  the  declination  is  said  to  be  to 
the  east.  When  it  points  to  the  west  of  true  north,  the  declination  is 
said  to  be  to  the  west. 

A  line  extending  along  the  earth  where  the  needle  points  to  the  true 
north,  is  called  a  line  of  no  declination.  Such  a  line  extends  from  near 
Cleveland,  Ohio,  to  Charleston,  South  Carolina. 

This  line  of  no  deviation  is  moving  to  the  westward  at  a  rate  that 
would  carry  it  around  the  earth  in  about  one  thousand  years.  This  is 
the  most  singular  of  all  the  phenomena  of  terrestrial  magnetism. 

For  all  parts  of  the  United  States  east  of  this  line,  the  declination  of 
the  needle  is  to  the  west ;  for  all  points  to  the  west  of  it,  the  declina- 
tion is  to  the  east.  The  north  end  of  the  needle,  at  all  places,  is  in- 
clined toward  the  line  of  no  declination. 

There  are  two  lines  of  no  declination,  eastern  and  western.     In  pro- 


MAGNETISM.  343 

ceeding  either  west  or  east  from  either  of  these  lines,  the  declination 
of  the  needle  gradually  increases,  and  becomes  a  maximum  at  a  certain 
intermediate  point  between  them.  These  two  lines  of  no  declination, 
in  the  present  age,  extend,  one  obliquely  over  North  America  and  the 
Atlantic  Ocean,  and  the  other  through  the  middle  of  China  and  across 
New  Holland,  and  they  are  supposed  to  communicate  near  both  poles 
of  the  earth. 

The  position  of  the  northern  magnetic  pole  is  about  19°  from  the 
north  geographical  pole  of  the  earth. 

639.  Daily,  annual,  and  other  variations  of  the  needle. 

— It  is  found  by  observation,  that  there  is  a  daily  variation  of  the 
needle  from  east  to  west  and  from  west  to  east,  averaging  a  little  less 
than  one  degree ;  caused,  doubtless,  by  the  action  of  the  sun,  and,  there- 
fore, this  variation  varies  in  different  latitudes. 

At  London  the  north  pole  of  the  needle  moves  westward  from  eight 
A.M.  until  one  P.M.  Soon  after  one  o'clock  it  begins  to  move  eastward, 
and  reaches  its  former  position  about  ten  P.M.  During  the  night  a 
small  oscillation  occurs,  the  north  pole  moving  westward  until  three 
A.M.,  then  returning  as  before. 

Other  variations  of  the  needle. — There  are  annual  variations 
of  the  needle,  conforming  to  the  movement  of  the  sun  in  the  solstices. 

There  are  irregular  variations  of  the  needle,  connected  with  the 
aurora  borealis,  or  other  cosmical  phenomena,  which  have  been  called 
magnetic  storms. 

The  magnetic  needle  also  deviates  more  or  less  by  the  near  approach 
of  masses  of  iron. 

640.  Inclination  or  dip  of  the  magnetic  needle.— Besides 

the  several  kinds  of  variations  already  noticed,  there  is  another,  called 
inclination  or  dip.  If  a  perfectly  unmagnetized  needle  be  suspended  at 
its  centre,  by  a  fibre  of  raw  silk,  it  will  remain  horizontal,  and  con- 
tinue to  point  in  any  horizontal  direction  in  which  it  may  be  placed. 
But  if  the  same  needle  be  magnetized  and  again  suspended  in  the  same 
manner,  it  will  not  only  assume  the  magnetic  north  and  south  direc- 
tion, but  it  will  also  assume  an  inclined  direction ;  that  is,  its  north 
pole  will  point  more  or  less  downward  toward  the  north  magnetic  pole 
of  the  earth,  and  the  south  pole  upward.  This  inclination  is  called 
the  dip. 

The  dip,  like  the  declination,  is  subject  to  continual  and  progressive 
changes.  At  London,  in  1576,  it  was  71°  50';  in  1723  it  was  at  its 
maximum,  being  74°  42',  while  now  it  is  only  68°  15';  showing  it  has 
decreased  about  3'  per  annum  for  the  last  hundred  and  fifty  years. 


344 


MAGNETISM. 


641.  Figure  58. — Action  of  the  earth  illustrated  by  the 
action  of  a  magnet. — The  magnetic  bar,  SN",  is  placed  horizontally 
on  the  diameter  of  a  semicircle,  representing  an  arc  of  the  meridian. 


FIG.  58. 


If  a  needle,  free  to  move  vertically,  be  placed  at  the  magnetic  equator, 
F,  its  two  poles  will  be  equally  attracted  by  the  magnet,  and  the  needle 
will  coincide  with  the  horizon,  shown  by  the  dotted  line;  its  real  north 
pole  standing  in  the  direction  of  the  south  pole  of  the  magnet. 

If  the  needle  be  placed  at  E,  its  N  pole  will  point  toward  the  south 
FlG   59  pole  of   the  magnet,  as  shown   by  the 

letters.  If  it  be  placed  at  H,  its  S  pole 
will  be  turned  toward  the  north  pole  of 
the  magnet,  as  indicated  by  the  letters. 

642.  Figure  59.— Dipping- 
needle. — To  show  the  dip,  and  to  meas- 
ure it  at  different  places,  a  needle  is  so 
mounted  as  to  be  perfectly  free  to 
move  or  rotate  in  a  vertical  plane  ;  the 
amount  of  dip  being  indicated  by  a 
graduated  circle  or  quadrant,  as  repre- 
sented in  the  figure.  At  any  place,  the 
dip  will  be  greatest  possible  when  the 
needle  vibrates  in  the  plane  of  the 
magnetic  meridian  (638). 

The  dip  varies  in  passing  from  place 
to  place,  being  greatest  at  the  magnetic 
poles,  and  nothing  at  the  magnetic 
equator,  as  clearly  illustrated  by  the 
following  diagram. 


MAGNETISM.  345 

643.  Figure  60.— Position  of  the  dipping-needle  in 
different  parts  of  the  earth.— SEN  represents  the  magnetic 
meridian,  and  ME  the  magnetic  equator  of  the  earth.  Let  S,  at  one 
extremity  of  the  line  SEN,  represent  the  north  magnetic  pole,  and  N, 
at  the  other  end  of  this  line,  the  south  magnetic  pole ;  and  the  several 
arrows,  the  dipping-needle,  with  the  north  pole  at  the  point  of  the 
needle. 

The  angle  which  the  needle  makes  with  the  horizon,  at  any  place,  is 
called  the  dip  at  that  place. 

At  the  equator  (that  is,  at  the  magnetic  equator),  it  will  be  seen 

FIG.  60. 


that  the  needle  assumes  the  horizontal  position,  being  equally  attracted 
by  the  two  magnetic  poles  of  the  earth. 

At  the  magnetic  poles  the  dip  is  90°,  the  arrows  being  directed  toward 
the  magnetic  poles.  When  nearer  the  north  pole  than  the  south,  the 
point)  or  N  pole,  of  the  needle,  points  to  the  magnetic  pole ;  but  when 
nearer  the  south  pole  than  the  north,  the  feather  end,  or  S  pole,  of  the 
needle,  points  to  the  magnetic  pole. 

Hence,  a  needle,  horizontally  balanced  at  the  equator,  would  dip 
toward  the  earth  as  it  is  carried  toward  the  north  pole,  and  vice  versa 
if  carried  toward  the  south  pole.  Mariners'  compasses,  therefore,  are 
provided  with  a  sliding  weight,  with  which  to  keep  the  needle  balanced 
in  different  latitudes ;  and  which  can  be  shifted  to  the  other  side  of 
the  pivot,  after  crossing  the  equator. 


346  MAGNETISM. 

644-  Figure  61. — The  mariner's  compass  is  arranged  in 
a  box  called  a  binnacle.  The  magnetic  needle,  delicately  poised  on  a 
socket  of  agate,  is  attached  to  the  lower  side  of  a  card,  on  which  is 


FIG.  61. 


printed  the  star  of  thirty-two  points ;  the  cardinal  points  being  N.  S. 
E.  W.  The  compass-box,  H,  is  hung  on  points,  called  gimbals,  one  of 
which  is  seen  at  H,  and  two  others  at  the  tops  of  the  arms  LL ;  the 
whole  being  firmly  secured  to  the  support  AA. 

By  this  method  of  supporting  the  compass-box,  it  always  remains 
horizontal,  however  the  ship  may  roll. 

Tables  for  correcting  variations  of  the  compass.— For 

most  practical  operations,  as  in  navigation  and  surveying,  the  devia- 
tion of  the  needle  from  the  true  north  and  south  is  taken  into  account, 
and  a  rule  of  corrections  applied.  The  amount  of  variation,  east  or 
west,  for  different  localities,  may  be  ascertained  from  tables  accurately 
calculated  and  arranged  for  this  purpose. 

Discovery  of  the  compass. — It  is  claimed  that  the  directive 
tendency  of  the  magnet  was  known  to  the  Chinese  some  2,000  years 
before  the  Christian  era;  but  it  was  not  known  to  the  European  na- 
tions until  about  ],250  years  after  the  Christian  era.  The  compasses 
of  that  time  were  merely  pieces  of  lodestone  fixed  to  a  cork,  which 
floated  on  the  surface  of  water ;  or  a  simple  sewing-needle,  rendered 
magnetic,  thrust  through  a  cork  or  reed,  and  placed  on  water. 


MAGNETISM.  347 

645.  Magnetic  intensity  varies  in  different  parts  of  the  earth. 
In  general,  it  is  greatest  about  the  poles  and  the  least  intense  about  the 
equator.     The  relative  intensity  of  different  points  is  determined  by 
the  use  of  the  needle  of  oscillation. 

The  greater  the  intensity  the  more  rapidly  will  the  needle  oscillate ; 
the  relative  intensity  being  as  the  square  of  the  numbers  of  oscilla- 
tions. This  method  of  testing  the  relative  intensity  of  terrestrial 
magnetism,  is  analogous  to  that  of  testing  the  force  of  gravity  by  the 
oscillations  of  the  pendulum  (61). 

646.  The  inductive  power  of  the  earth's  magnetism  is 

manifested  by  the  polarity  of  bars  of  iron  and  steel,  which  have  been 
standing  for  a  long  time  in  a  vertical  position  (624). 

647.  Utilization  of  magnetism. — Its  directive  power  renders 
the  compass  invaluable  to  the  explorer  of  the  wilderness,  to  the  navi- 
gator, to  the  surveyor,  and  the  miner.    The  mineralogist  and  the  gen- 
eral investigator  find  it  indispensable  in  many  researches.     Different 
artisans  make  valuable  use  of  the  attractive  force  of  magnets. 


348  ELECTRICITY. 


CHAPTEK   XIY. 

(CHART  NO.  8.) 

ELECTEICITY. 
STATICAL     OR     FRICTIONAL     ELECTRICITY. 

Fundamental  Principles. 

648.  Definitions. — The  name  electricity  is  derived  from  the 
Greek  elektron,  which  means  amber. 

Electricity  is  an  imponderable  agent,  existing  in  all  substances 
throughout  nature,  without  affecting  their  volume  or  their  tempera- 
ture, or  giving  any  indication  of  its  presence  when  in  a  latent  or  quiet 
state.  When,  however,  it  is  by  some  means  liberated  from  this  repose, 
it  is  capable  of  producing  the  most  sudden  and  destructive  effects,  or 
of  exerting  powerful  influences  by  a  gentle  and  long-continued  action. 

Electricity,  as  a  science,  treats  of  the  excitation,  the  manifestations, 
and  the  effects  of  this  agent. 

649.  Discovery  of  electricity. — The  ancients,  six  hundred 
years  before  the  Christian  era,  knew  that  amber,  when  rubbed,  would 
attract  small  pieces  of  straw,  barbs  of  quills,  and  the  like.    Beyond 
this  fact,  which  remained  without  value  for  more  than  two  thousand 
years,  nothing  was  known  on  the  subject  until  the  end  of  the  sixteenth 
century.     The  success  with  which  this  subtle  agent  is  handled  and 
controlled  at  the  present  day,  is  shown  by  a  large  group  of  sciences  to 
which  it  has  given  birth,  and  by  the  existence  of  the  Atlantic  Cable, 
and  countless  telegraphic  wires  stretching  around  the  world,  annihilat- 
ing time  and  space,  and  enabling  us  to  converse  with  our  antipodes. 
Yet  the  exploration  of  this  vast  field  of  science  is  but  just  commenced. 

650.  The  sources  of  electricity.— The  chief  sources  of  elec- 
trical excitement  are — 1st.  Friction  of  dry  substances ;  2d.  Chemical 
action,   or   chemical   composition   and   chemical   decomposition ;    3d. 
Magnetism,)  producing  magneto-electricity ;  4th.  Heat,  or  thermo-elec- 
tricity; 5th.  Animal  electricity ;  6th.  Electricity  of  Plants. 

651.  Figure  1. — Electrical  effects. — If  a  dry  and  warm  glass 
rod  or  tube  be  briskly  rubbed  with  cat's  fur,  or  a  piece  of  silk  or  woolen 


ELECTRICITY.  349 

cloth,  it  attracts  to  itself  bits  of  paper,  shreds  of  cotton,  gold  leaf, 
feathers,  pith,  and  other  light  substances,  holding  them  for  an  instant, 
and  then  repelling  them,  as  illustrated  by  the  figure. 

If  the  experiment  be  performed  in  the  dark,  a  feeble  bluish  light  is 
seen  in  the  path  of  the  rubber.     If  the  glass  is  immediately  presented 

FIG.  1. 


to  a  metallic  body,  or  to  the  knuckle  of  the  finger,  a  purple  spark  will 
dart  off  from  the  glass  with  crackling  sound.  If  the  glass  be  held  near 
the  face,  a  sensation  is  experienced  similar  to  that  produced  by  draw- 
ing a  fine  thread  across  the  skin.  The  same  effects  are  produced  by 
the  rubber.  A  peculiar  odor  accompanies  electrical  excitement,  as  also 
a  peculiar  taste,  if  the  electricity  be  excited  by  voltaic  action. 

Bodies  thus  excited  are  said  to  be  electrified)  a  condition  which  is 
only  transient. 

These  simple  experiments  contain  the  germ  of  electrical  science 
(671). 

652.  Electroscope. — Electrical  pendulum.— An  electroscope 
(of  which  there  is  a  variety)  is  an  apparatus  to  show  whether  or  not  a 
body  is  electrified.  The  most  simple  of  these  is  the  electrical  pendulum, 
which  consists  simply  of  a  small  ball  of  elder-pith,  or  cork,  suspended 
by  a  fine  silk  thread,  which  is  fastened  at  the  upper  end  to  a  stem  of 
copper  provided  with  a  glass  support. 

If  an  electrified  body  be  presented  to  the  pendulum,  the  pith-ball 
will  be  attracted  by  it.  If  the  body  is  not  electrified,  the  ball  will  not 
move. 

When  very  sensitive  tests  are  required,  more  delicate  instruments, 
called  electrometers,  are  employed. 


350 


ELECTRICITY. 


FIG.  2. 


653 .  Figure  2. — Vitreous  and  resinous,  or  positive  and 
negative,  electricities. — There  are  two  kinds  of  electricity,  the 
difference  between  them  depending  upon  the  kind  of  material  which  is 
subjected  to  friction. 

If  a  tube  of  glass,  shown  in  the  figure,  be  rubbed  with  a  piece  of  silk, 
and  then  presented  to  the  electrical  pendulum,  the  pith-ball  will  be 
attracted  and  then  repelled. 

If,  now,  a  stick  of  sealing-wax,  as  shown,  be  rubbed  with  flannel,  and 
brought  near  the  pith-ball  (which  is  already  charged  with  the  elec- 
tricity from  the  glass),  it  will  be 
attracted  to  the  sealing-wax, 
though  it  was  repelled  by  the 
glass.  If  the  wax  be  presented 
to  the  pith-ball  first,  the  ball  will 
be  attracted  to  the  wax,  become 
charged  with  its  electricity  and 
then  repelled.  In  this  state  the 
ball  will  be  attracted  by  the 
glass. 

A  pith-ball  suspended  between 
the  glass  and  wax,  as  represented, 
will  continue  to  swing  back  and 
forth  from  one  to  the  other,  as 
indicated  by  the  arrows,  as  long 
as  there  is  sufficient  electrical 
excitement  to  charge  it. 

The  ball  being  charged  by  the  glass,  is  then  repelled  and  attracted 
by  the  wax.  The  wax  absorbs  the  electricity  brought  from  the  glass, 
and  charges  it  with  its  own  ;  when  it  will  be  repelled  by  the  wax  and 
attracted  by  the  glass ;  and  so  on. 

This  shows  that  the  action  of  electricity  developed  in  glass  and  in 
resin  is  different ;  the  one  repelling  when  the  other  attracts. 

The  electricity  developed  by  rubbing  glass  is  called  vitreous  or 
positive  electricity ;  that  developed  by  rubbing  resin  or  sealing-wax  is 
called  resinous  or  negative  electricity. 

Glass  and  resin  are  but  types  of  two  large  classes  of  substances,  which 
possess  more  or  less  perfectly  this  characteristic  difference,  as  above  ex- 
plained. 

654.  The  theory  of  two  fluids.— This  theory  is  based  upon 
the  supposition  that  two  electrical  fluids  exist,  in  unexcited  bodies, 
in  a  state  of  combination,  forming  what  is  called  a  neutral  fluid.     This 
neutral  fluid  has,  of  itself,  no  obvious  properties.    Hence,  bodies  which 


ELECTRICITY.  351 

only  contain  it  are  said  to  be  neutral.  The  earth  is  considered  a  great 
reservoir  of  this  fluid. 

If  this  neutral  fluid  is  decomposed,  and  the  two  fluids  separated,  by 
whatever  means,  then  various  electrical  phenomena  are  immediately 
manifested. 

If  glass  is  rubbed  with  silk,  the  positive  fluid  goes  to  the  glass  and 
the  negative  to  the  silk.  But  if  sealing-wax  is  rubbed,  the  negative 
fluid  goes  to  the  sealing-wax  and  the  positive  to  the  silk. 

All  electrical  phenomena  are  supposed  to  be  due  to  the  tendency  of 
the  two  fluids  to  reunite  and  neutralize  each  other. 

655.  The  single  fluid  hypothesis  is  simple,  was  for  a  long 
time  adopted,  and  will  account  for  most  of  the  phenomena.  It  suppo- 
ses the  existence,  throughout  all  space,  of  a  subtle  and  exceedingly  elas- 
tic fluid,  called  the  electric  fluid  ;  that  it  is  repulsive  of  its  own  parti- 
cles, but  attracted  by  particles  of  other  matter;  that  all  bodies  contain  a 
specific  quantity  of  it;  and  that,  when  thus  combined  with  other 
matter,  it  loses  its  self-repellent  tendency.  In  its  natural  state  every 
substance  has  exactly  its  own  quantity  of  this  fluid,  and  is  conse- 
quently in  a  state  of  electrical  indifference.  If  electrical  excitement 
is  developed  in  a  body,  by  whatever  means,  this  electrical  equilibrium 
is  disturbed ;  and  the  body  becomes  positively  electrified  when  it  is 
charged  with  more  than  its  specific  or  natural  quantity,  and  negatively 
electrified  when  it  is  deprived  of  a  portion  of  its  specific  or  natural 
quantity. 

Hence,  bodies  positively  electrified  are  restored  to  equilibrium  by  part- 
ing with  the  excess,  and  bodies  negatively  electrified,  by  receiving  from 
surrounding  bodies  sufficient  to  satisfy  the  deficiency. 

This  hypothesis  is  strikingly  similar  to  that  commonly  accepted  in 
explanation  of  the  equilibrium  of  heat. 

On  the  principles  of  either  of  these  hypotheses,  it  is  impossible  to 
produce  one  kind  of  electricity  without  the  other  simultaneously 
appearing.  The  positive  and  negative  must  be  always  co-ordinately 
generated. 

The  term  fluid  is  calculated  to  convey  an  erroneous  idea,  for  it  is 
employed  only  as  a  convenient  expression  for  an  unknown  cause.  In- 
stead of  assuming  the  existence  of  a  separate  fluid  or  ether,  as  a 
medium  for  light,  heat,  or  magnetic  electricity,  it  is  more  in  accord- 
ance with  sound  philosophy  to  suppose  that  these  separate  manifesta- 
tions are  only  different  functions  of  the  one  ethereal  medium,  which 
fills  the  entire  universe,  and  from  whose  correlations  to  the  particles 
of  matter,  all  physical  phenomena  proceed. 


352  ELECTRICITY, 

656.  Figure  3.— Attraction  and  repulsion.— If  two  pith- 
balls  be  suspended,  as  shown  in  the  figure,  and  both  charged  alike,  that 
FIG.  3.  is>  with  either  positive  or  negative 

electricity,  they  will  be  mutually 
repelled,  as  represented  by  the 
dotted  balls  and  the  signs  plus  and 
minus.  But  if  one  ball  be  charged 
with  positive  and  the  other  with 
negative  fluid,  as  indicated,  they 
will  attract  each  other. 

These  simple  experiments  show 
a  similarity  between  these  actions 
and  the  law  of  magnetic  attrac- 
tions and  repulsions  (626). 

657.  Laws  of  electrical 
attraction  and  repulsion.— 

The    following    laws    have   been 
deduced  from  theory  and  confirmed  by  experiment : 

1.  Fluids  of  the  same  name  repel  each  other,  and  fluids  of  opposite 
names  attract  each  other. 

2.  The  intensities  of  the  attractions  and  repulsions  vary  inversely 
as  the  square  of  the  distances  betiueen  them. 

3.  The  distances  remaining  the  same,  the  attractions  and  repulsions 
are  directly  as  the  quantities  of  electricities  possessed  by  the  two  bodies. 

658.  Conductors  of  electricity. — Conductors,  or  conducting 
substances,  are  those  which  permit  electricity  to  pass  through  them. 

Some  bodies,  electrically  excited,  part  with  their  excitement  instantly, 
others  slowly,  depending  on  the  nature  of  the  substance  excited,  and 
of  those  with  which  it  is  brought  in  contact.  As  bodies  differ  very 
much  in  their  power  to  conduct  electricity,  they  are  divided  into 
classes,  called  good  and  bad  conductors,  or  conductors  and  non-con- 
ductors. 

Good  conductors  propagate  the  excitement  to  all  parts  of  their  sur- 
faces ;  and,  when  in  contact  with  the  earth,  part  with  it  as  quickly  as 
they  receive  it. 

Among  the  good  conductors  are  the  following  substances,  placed  in 
the  order  of  their  conducting  power.  The  metals — silver  and  copper 
standing  first,  lead  and  quicksilver,  last — charcoal,  plumbago,  coak, 
hard  anthracite,  acids,  saline  solutions,  water,  snow,  living  things, 
flame,  smoke,  vacuum,  vapors  of  alcohol  and  ether,  earth  and  moist 
rocks,  etc. 


ELECTRICITY.  353 

Bad  conductors  receive  and  part  with  electricity  very  slowly ;  conse- 
quently they  retain  free  electricity  for  a  long  time,  and  obstruct  its 
passage  from  one  body  to  another. 

Among  the  best  non-conductors  are  resins,  gums,  India-rubber,  silk, 
glass,  precious  stones,  spirits  of  turpentine,  oils,  air,  and  dry  gases. 

659.  Insulators.  —  Insulators   are  non-conducting  substances, 
placed  between  bodies  to  be  electrified  and  the  earth  and  other  sur- 
rounding bodies,  to  prevent  the  passage  of  the  electricity.     A  body, 
therefore,  is  said  to  be  insulated  when  it  is  supported,  and  cut  oif  from 
surrounding  bodies,  by  good  non-conductors.     Insulators  are  usually 
made  of  glass.      Gutta-percha  and  whalebone-rubber  are  among  the 
best  insulators  known. 

660.  The  earth  is  the  reservoir  into  which  all  electrical  ex- 
citements are  returned.     The  air,  unless  saturated  with  moisture,  is  a 
poor  conductor ;  hence  it  serves  to  insulate  the  earth,  which  is  a  good 
conductor. 

Except  for  the  non-conducting  property  of  the  air,  all  electrical  phe- 
nomena would  have  remained  invisible  and  unobserved. 
It  should  be  Icept  in  mind  that  the  earth  is  always  negatively  excited. 

661.  Method  of  electrifying  bodies.— In  order  to  electrify  a 
conducting  body,  it  must  first  be  insulated  from  the  earth  and  sur- 
rounding bodies,  by  placing  it  upon  some  sort  of  a  glass  or  other  non- 
conducting support.     Thus  supported,  it  must  be  rubbed  by  an  insu- 
lated rubber.     Conductors  may  be  electrified  also  by  contact  and  by 
induction. 

If  the  body  and  rubber  are  not  insulated,  the  excitement  or  fluid  will 
pass  to  the  earth  through  the  support  and  body  of  the  operator,  as  fast 
as  generated. 

Non-conducting  bodies  are  only  electrified  by  friction. 

The  method  of  electrifying  by  contact  depends  upon  the  conducti- 
bility  of  the  body.  If  a  conducting  body  be  brought  in  contact  with 
an  electrified  body,  a  portion  of  the  electricity  of  the  excited  body  flows 
into  the  unexcited  body.  If  the  two  bodies  are  exactly  alike,  the  elec- 
tricity will  be  equally  distributed  over  both.  If  they  differ  in  size  or 
shape,  the  electricity  will  not  be  equally  distributed. 

Electrifying  bodies  by  induction  is  performed  in  a  manner  similar  to 
that  of  magnetizing  bodies  by  induction,  as  will  be  hereafter  explained. 

662.  Electrical  tension  is  a  condition  ol  constrained  equilib- 
rium, and  when  the  fluids  or  electricities,  to  which  it  is  due,  reunite, 

23 


354:  ELECTRICITY. 

an  electrical  current  is  produced  from  the  reaction  of  the  opposing 
fluids,  analogous  to  mechanical  motion  from  the  recoil  of  a  spring. 
The  energy  with  which  they  reunite,  when  communication  is  made 
between  them,  shows  the  state  of  tension  in  which  they  existed. 

All  electrified  bodies  manifest  electrical  tension,  and  attract  other 
bodies,  decomposing  their  natural  electricity,  drawing  from  them  a 
portion  of  the  opposite  fluid. 

663.  Figure  4.  — Electricity  accumulates  only  on  the 
outer  surfaces  of  bodies. — If  a  body  be,  however,  thoroughly 
FlG  4  charged,  the  fluid  does  not 

penetrate  the  substance  of 
the  body;  and  if  it  be  a 
hollow  body,  it  does  not 
even  reside  on  the  inner  sur- 
face, but  accumulates  wholly 
on  the  outer  surface. 

If  a  copper  or  other  metal 
sphere,  mounted  on  a  glass 
or  insulated  support,  S,  be 
electrified,  and  then  covered 
with  the  thin  hemispheres, 
L  and  N,  made  of  the  same 
metal,  and  provided  with 
insulating  handles,  it  will 
be  found,  on  removing  the 
covers,  that  they  are  electrified,  and  the  sphere  is  deprived  of  every 
trace  of  electrical  excitement. 

This  is  due  to  the  repulsive  power  of  the  fluid  within  driving  the 
excitement  to  the  surface,  where  it  meets  the  non-conducting  air  and 
is  arrested ;  and  also  to  the  inductive  influence  of  the  electricity  of  sur- 
rounding bodies  and  of  the  walls  of  the  room. 

Figure  5.— That  electricity  accumulates  only  on  the 
surface,  shown  in  a  different  way. — This  figure  represents  a  ribbon, 
T,  of  metallic  paper,  wound  around  a  metallic  axis,  insulated  with 
silk  threads;  two  sets  of  pith -balls  are  suspended  by  linen  threads 
at  the  lower  end  of  the  ribbon.  If  the  ribbon  is  wound  up,  by  the 
insulating  crank,  and  the  whole  apparatus  is  electrified,  the  pith-balls 
diverge  powerfully.  If  the  ribbon  is  now  unwound,  by  drawing  the 
insulating  string  at  the  bottom,  the  pith-balls  gradually  fall,  and 
finally  come  almost  in  contact.  But  as  the  ribbon  is  again  wound  up, 
the  balls  diverge  as  before.  This  may  be  repeated  several  times. 


ELECTRICITY. 


355 


PIG.  5. 


As  the  surface  increases  the  electricity  is 
spread  out ;  as  the  surface  is  diminished,  it 
is  concentrated  and  intensified ;  thus  illus- 
trating the  relation  of  surface  and  intensity. 

It  is  thus  proved  that  all  the  electricity 
with  which  a  conducting  body  is  charged,  is 
disposed  on  its  surface. 

Hence,  a  ball  of  wood  or  pith,  covered  with 
tin-foil  or  gold-leaf,  can  accumulate  on  its 
surface  as  much  electricity  as  if  it  was  of 
solid  metal.  A  hollow  and  solid  sphere,  of 
the  same  size  and  material,  will  be  charged 
with  exactly  the  same  quantity  of  electricity. 


664-  Proof-plane. — The  proof-plane 
is  an  instrument  for  determining  the  rela- 
tive quantities  of  electricity  that  are  found 
on  the  different  parts  of  an  electrified  conductor.  It  consists  of  a  disk 
of  gilt  paper,  attached  to  the  end  of  an  insulating  rod,  as  gumlac,  shown 
in  the  hand  of  Fig.  6.  The  rod  is  held  in  the  hand,  the  disk  applied  to 
different  parts  of  the  electrified  surface,  and  after  each  contact  it  is 
presented  to  the  electrical  pendulum. 

FIG.  6. 


665.  Figure  6.— Distribution  dependent  on  form.— The 

distribution  of  electricity  over  the  surface  of  bodies  depends  upon  their 
form.    If  the  body  be  a  sphere,  the  distribution  is  uniform.     If  the 


356  ELECTRICITY. 

proof-plane  be  applied  at  different  parts  of  an  excited  ellipsoid,  like  the 
figure,  it  will  be  found  that  the  electrical  fluid  is  not  equally  distrib- 
uted on  all  parts  of  the  surface.  The  maximum  is  found  at  L,  and 
the  minimum  at  F ;  showing  a  tendency  in  electrical  excitement  to 
accumulate  about  the  extremities  of  solids  having  unequal  axes. 

In  cylinders,  the  concentration  of  force  occurs  not  far  from  each  end, 
and  is  feeble  in  the  middle.    In  plates,  the  maximum  is  near  the  edges. 

666.  The  power  of  points.— On  the  principle  just  explained, 
the  power  of  points,  in  concentrating  electricity,  produces  a  tension 
sufficient  to  overcome  the  resistance  of  the  air,  causing  it  to  pass  off  as 
rapidly  as  it  accumulates,  to  the  nearest  bodies,  or  into  the  air,  in  an 
electrical  brush  or  pencil,  visible  in  the  dark. 

667.  The  loss  of  electricity  in  excited  bodies  is  constant, 
chiefly  from  two  causes  :  1st,  the  moisture  of  the  air ;  and,  2d,  from  the 
imperfection  of  the  insulation,  even  when  the  best  insulators  are 
employed. 

INDUCTION     OF     ELECTRICITY. 

668.  Figure  7. — Bodies  electrified  by  induction. — E  is  a 

prime  conductor  of  an  electrical  machine  ;  NAP,  a  metallic  cylinder, 
insulated  by  a  rod  or  support  of  glass,  having  several  pairs  of  pith-balls 
attached  to  its  lower  surface. 

FIG.  7. 


If  the  prime  conductor  be  charged  with  positive  electricity,  and 
placed  within  a  few  inches  of  the  cylinder,  the  pith-balls  near  the  con- 
ductor, E,  will  be  attracted  and  separated ;  those  at  the  opposite  end 


ELECTRICITY.  357 

will  be  repelled  and  separated ;  while  those  in  the  central  part  will  not 
be  repelled,  attracted,  or  separated.  If  the  prime  conductor  be  re- 
moved, the  electroscopes,  or  balls,  will  cease  to  indicate  any  excite- 
ment. 

The  explanation  of  these  facts  is,  that  the  neutral  fluid  of  the  cylin- 
der has  been  decomposed  by  the  influence  of  the  prime  conductor ;  the 
positive  ( + )  fluid  being  repelled  to  P,  and  the  negative  ( — )  fluid  at- 
tracted to  N,  while  at  the  central  part,  A,  a  neutral  point  is  found. 
When  the  prime  conductor,  the  disturbing  cause,  is  removed,  the  two 
electricities  of  NAP  unite  again,  leaving  the  cylinder  entirely  passive; 
showing  that  none  of  the  electric  fluid  is  transferred  from  the  prime 
conductor  to  the  cylinder. 

Bodies  thus  affected  are  said  to  be  electrified  by  induction. 

669.  Figure  8.— The  two  fluids  separated  and  obtained 
by  induction. — Let  three  insulated  cylinders  be  placed  in  a  row,  and 
in  contact,  as  shown  in  the  figure ;  approach  the  positively  electrified 
prime  conductor,  E,  toward  the  cylinder,  N.  By  induction  (668),  the 

FIG.  8. 


neutral  electricity  of  the  three  cylinders  will  be  decomposed ;  the  nega- 
tive (  — )  being  attracted  to  and  accumulated  in  N,  and  the  positive 
( 4- )  repelled  to  and  accumulated  in  P.  Whilst  in  this  condition,  re- 
move the  cylinders,  P  and  N,  at  a  distance  from  E,  and  from  each 
other.  The  separated  electricities  will  thus  be  kept  from  uniting ;  and, 
upon  testing,  N  will  be  found  negatively,  and  P  positively,  electrified, 
as  indicated  by  the  signs  plus  and  minus. 

By  these  principles  of  induction  a  great  variety  of  electrical  pheno- 
mena are  easily  explained. 

The  laws  of  electrical  induction  are  thus  stated : 

1  st.  A  body  electrized  by  induction,  possesses  no  more  electricity  than 
before. 

3d.  If  a  conductor,  electrized  by  induction,  is  touched,  or  made  to  com- 
municate witli  the  earth  in  any  part  of  its  surface,  it  parts  with  a  por- 
tion of  its  electricity,  always  of  the  same  name  with  the  electrifying 
body,  and  it  retains  the  fluid  of  the  opposite  name. 


358  ELECTRICITY. 

670.  Figure  9.— Dielectrics.— Explanation  of  induction. 

— Induction  takes  place  at  a  distance  by  polarizing  the  molecules  of 
the  intervening  non-conductor.  Because  air,  and  other  non-conductors, 
permit  the  passage  of  electrical  influence  in  this  manner,  they  are  called 
dielectrics,  in  distinction  from  electrics. 

Therefore,  in  the  above  experiments  (668-9),  the  disturbance  of  the 
natural  electric  state  of  the  cylinders  is  not  produced  by  action  at  a 
distance.  It  takes  place  through  the  medium  of  the  intervening  air  (a 

FIG.  9. 


dielectric).  Thus,  if  P  (Fig.  9)  be  the  prime  conductor,  and  N  the 
cylinder,  let  the  small  circles  between  them  represent  molecules  of  air 
(or  any  other  intervening  dielectric  medium),  and  they  will  all  be 
polarized,  as  it  is  termed,  having  their  negative  (— )  parts  or  poles 
turned  toward  the  positively  excited  body  P,  and  their  positive  (  +  ) 
parts  or  poles  toward  the  cylinder  N,  which  will  attract  the  negative 
(  — )  fluid  of  the  cylinder  to  the  end  nearest  to  the  positive  ( +  )  prime 
conductor. 

Dielectrics  differ  in  their  specific  inductive  capacity.  If  air  (the 
lowest)  be  1,  the  following  will  stand  thus :  air,  1 ;  resin,  1.77 ;  pitch, 
1.80 ;  wax,  1.86 ;  glass,  1.90  ;  sulphur,  1.93 ;  shellac,  1.95. 

671.  Attraction  and  repulsion  of  light  bodies  (651)  can 
be  accounted  for  only  by  the  laws  of  induction,  as  above  explained  (668, 
669,  670).     The  excited  glass  or  resin  decomposes  the  neutral  elec- 
tricity of  the  bits  of  paper  or  pith-balls,  repelling  the  electricity  of  the 
same  name,  which  leaves  them  with  an  opposite  excitement  to  the 
glass  or  resin,  and,  therefore,  they  become  attracted  to  the  electrified 
body. 

Electrometers. 

672.  Electrometers. — The  electroscope,  previously  described 
(652),  serves  only  to  indicate  the  presence  and  name  of  the  electricity, 
but  not  the  quantity.     Electrometers  are  measures  of  electric  force  or 
intensity,  and  depend  upon  the  principle,  that  like  kinds  of  electricity 
repel  and  opposite  kinds  attract. 

The  two  corks  or  pith-balls,  suspended  by  linen  threads  (656)  are 


ELECTRICITY. 


359 


one  of  the  most  simple  of  these  contrivances.    The  distance  to  which 
they  will  diverge  is  a  rough  measure  of  the  intensity  of  the  electric 

force. 

FIG.  10. 

673.  Figure  10.— The  quadrant  electrom- 
eter consists  of  a  slender  stem  or  support  of  baked 
wood,  to  which  is  affixed  a  semicircular  piece  of 
ivory,  from  the  centre  of  which  hangs  a  pith-ball,  on 
a  small  arm  of  whalebone,  as  shown. 

If  this  instrument  is  placed  on  the  prime  conduc- 
tor (678),  or  other  electrified  body,  the  stem  partici- 
pating in  the  electricity,  and  repelling  the  pith-ball, 
as  indicated  by  the  arrow,  the  amount  of  repulsion 
may  be  read  off  on  the  graduated  semicircle.  The 
number  of  degrees  do  not  express  the  true  electrical 
intensity ;  and  whatever  may  be  the  intensity,  the  ball 
cannot  be  repelle.d  beyond  90°. 

67 Jf. — Figure  11. — The  gold-leaf  electrometer  consists  of 
a  bell-jar,  provided  with  a  copper  rod,  passing  through  a  cork,  termi- 
nated with  a  copper  knob,  T,  at  FIG.  11. 
the    top,   and    sustaining    two 
strips    of   gold-leaf,   E,  placed 
face  to  face. 

If  the  knob,  T,  be  electrized, 
the  gold-leaves  diverge,  and  the 
extent  of  their  divergence, 
measured  on  a  graduated  arc, 
serves  to  show  the  intensity  of 
the  electricity.  Two  strips  of 
tin-foil,  L,  are  pasted  to  the 
inside  of  the  jar  to  discharge 
the  diverging  leaves,  when  they 
are  repelled  so  as  to  reach  the 
sides,  to  prevent  the  inside  of 
the  jar  from  becoming  electri- 
fied by  induction ;  otherwise  the  apparatus  would  be  useless.  To  avoid 
dampness,  the  air  within  is  dried  by  quick  lime,  and  the  top  of  the  jar 
and  cork  are  coated  with  an  insulating  varnish,  made  of  sealing-wax 
dissolved  in  alcohol. 

Instead  of  the  gold-leaves,  straws  and  pith-balls  are  sometimes  em- 
ployed. 

When  more  exact  measurement  of  electricity  is  required,  the  torsion 
electrometer  is  employed. 


360  ELECTRICITY. 

675.  Method  of  using  the  gold-leaf  electrometer  (Fig. 
11). — If  the  negatively  electrized  rod,  in  the  hand,  be  brought  near  the 
knob,  T,  it  will,  by  induction,  attract  the  positive  fluid,  and  repel  the 
negative  to  the  gold-leaves,  and  diverge  them,  as  indicated  by  the  signs 
plus  and  minus,  and  by  the  position  of  the  leaves. 

If,  now,  the  finger  be  applied  to  the  knob,  T,  the  positive  fluid 
passes  off,  through  the  body,  to  the  earth ;  but,  on  withdrawing  the 
finger,  the  leaves  diverge  under  the  influence  of  the  negative  fluid 
remaining  in  the  apparatus. 

If  the  rod  were  positively  electrified  the  leaves  would  receive  the 
positive  fluid ;  yet,  on  applying  the  finger  to  the  knob,  the  positive 
fluid  passes  off  to  the  earth  and  the  negative  remains. 

To  ascertain  the  kind  of  electricity  in  a  body,  proceed,  as  just  ex- 
plained, to  charge  the  leaves  with  negative  fluid,  then  approach  the 
body  to  be  tested,  to  the  knob.  If  the  body  is  negatively  electrified, the 
leaves  will  be  still  further  separated ;  if  positively  electrified,  the  leaves 
will  be  drawn  together. 


ELECTRICAL    MACHINES. 

676.  Figures  12  and  13. — The  electrophorus. — An  elec- 
trical machine  is  any  apparatus  by  which  electricity  may  be  generated, 
and  electrical  phenomena  obtained  at  pleasure. 

The  electrophorus,  or  carrier  of  electricity,  is  the  simplest  of  all  such 
FIG.  12.  devices.    It  consists  of  a  cake  of 

resin,  or  disk  of  whalebone  India- 
rubber,  W,  eight  or  ten  inches  in 
diameter,  and  a  wooden  plate,  S, 
covered  with  tin-foil,  and  pro- 
vided with  an  insulating  handle 
of  glass. 

To  use  this  instrument,  first 
excite  the  resinous  cake  by  vig- 
orously rubbing  it  with  cat's  fur  or 
warm  flannel,  which  developes 
negative  electricity  in  the  resin. 
Then  apply  the  disk,  S,  to  the 
resin,  as  shown  in  Fig.  12,  hold- 
ing it  by  the  handle.  The  cake 
of  resin  acts  upon  the  disk  by  induction,  drawing  its  positive  fluid  to 
the  tin-foil  on  the  lower  face,  and  repelling  its  negative  fluid  to  the 
foil  on  its  upper  face.  Now,  touch  the  finger,  as  shown  (Fig.  12),  to 
the  upper  face,  in  order  to  allow  the  negative  fluid  to  escape  into  the 


ELECTRICITY. 


361 


common  reservoir  (the  earth,  660).  If  the  disk  now  be  raised  by  its 
handle  from  the.  resinous  cake,  and  touched  with  the  knuckle,  as 
shown  in  Fig.  13,  a  spark  will  FIG.  13. 

pass,  which  is  due  to  the 
negative  electricity  passing 
from  the  body  of  the  experi- 
menter to  the  positively  elec- 
trified plate. 

If  the  air  is  dry  the  disk 
can  be  applied  to  the  resinous 
cake,  and  the  spark  evolved 
several  times  without  further 
rubbing. 

If  the  plate  be  left  in  repose 
on  the  resin,  the  apparatus 
will  remain  charged  even  for 
weeks.  And  the  Leyden  jar 
may  be  charged  with  the  instrument  at  any  time. 

If  the  disk,  S,  were  raised  from  the  resinous  cake,  W,  without  apply- 
ing the  finger,  as  represented  in  Fig.  12,  it  would  manifest  no  electrical 
excitement ;  the  two  fluids  reuniting,  as  in  the  insulated  conductor  (668). 

The  phenomena  involved  in  the  electrophorus  are  in  accordance  with 
the  laws  of  induction  (669). 

FIG.  14. 


677.  Figure  14. — The  cylinder  electrical  machine. — To 

obtain  larger  quantities   of  electricity  than  can  be  supplied  by  means 


362 


ELECTRICITY. 


already  described,  machines  of  various  sizes  and  forms  are  made.  How- 
ever these  may  differ,  there  are  at  least  three  essential  parts,  viz. :  1st,  a 
non-conductor,  usually  made  of  glass,  and  revolved  on  an  axis,  to  pro- 
duce friction  ;  3d,  a  rubber  to  press  on  the  conductor,  made  of  some 
soft,  elastic,  non-conducting  body,  as  a  cushion  of  leather,  peculiarly 
prepared ;  3d,  conductors  (one  or  two)  insulated  with  glass  supports, 
to  receive  the  electricity  as  it  is  generated. 

The  figure  represents  an  end  view  of  the  cylinder  electrical  machine. 
A  is  the  cylinder  of  glass,  which  may  be  from  six  inches  to  two  feet  in 
diameter,  and  once  and  a  half  to  twice  its  diameter  in  length,  revolved, 
by  means  of  a  winch,  between  two  upright  supports  of  wood,  dried  and 
varnished.  R  is  the  rubber,  somewhat  shorter  than  the  cylinder,  made  of 
wood  and  covered  with  leather,  under  which  are  several  thicknesses  of 
flannel,  and  covered,  over  all,  with  black  silk.  The  face  and  lower  side  of 
the  rubber  are  also  coated  with  an  amalgam,  composed  of  4  parts  mer- 
cury, 8  zinc,  and  2  tin,  mixed  with  some  unctuous  substance.  F  is  the 
prime  conductor,  placed  opposite  to  the  rubber,  and  consists  of  a  hol- 
low cylinder  of  brass  or  wood,  covered  with  tin-foil,  supported  upon  a 
glass  rod.  On  the  side  next  the  glass  it  is  furnished  with  points,  L, 
nearly  touching  the  cylinder,  A,  which  draw  off  the  electricity  from  the 
excited  glass  to  the  prime  conductor  (666). 

To  assist  in  preventing  the  escape  of  the  fluid,  an  apron  of  black  silk 
extends  from  the  rubber  over  to  the  points,  L.  The  prime  conductor 
should  be  made  as  smooth  as  possible. 

FIG.  15.  The    rubber    is    sometimes 

mounted  upon  an  insulated 
conductor,  for  the  purpose  of 
developing  negative  electricity. 
The  air  inclosed  in  the  cylin- 
der should  be  free  of  moisture. 
The  arrow,  S,  represents  the 
direction  in  which  the  cylinder 
is  turned,  and  the  flow  of  the 
fluid,  which  takes  place,  of 
course,  under  the  silk  apron. 
T  is  a  small  sphere  attached 
to  the  prime  conductor,  from 
which  the  fluid  is  drawn  in 
performing  experiments. 

678.  Figure  15.— The 
plate  electrical  machine. 

— In  this  machine  a  large  cir- 


ELECTRICITY.  363 

cular  plate  of  glass  is  substituted  for  the  cylinder,  and  is  usually  fur- 
nished with  four  rubbers,  two  of  which,  TT,  are  shown,  held  in  suitable 
tightening  clamps.  The  plate  is  revolved  between  these  rubbers  by 
turning  the  winch.  Extending  from  the  rubbers  are  silk  aprons,  SS; 
//"are  points  for  collecting  the  fluid  and  conveying  it  to  the  prime 
conductor,  P,  which  is  supported  by  the  glass  rod,  A.  F  is  an  elec- 
trometer. The  arrow  indicates  the  direction  in  which  the  plate  is 
revolved. 

These  machines  are  variously  modified,  and  some  are  made  with  two 
plates.  One,  at  least,  has  been  made  with  double  plates  six  feet  in 
diameter. 

679.  Use  of  the  electrical  machines.— When  the  plate  is 
revolved,  the  friction  developes  a  great  quantity  of  positive  electricity 
on  the  glass,  whilst  the  negative  fluid  goes  to  the  rubbers  and  is  con- 
veyed, through  the  frame,  to  the  common  reservoir,  the  earth,  and  so 
disappears.     The  neutral  fluid  on  the  conductors,  or  prime  conductor, 
is  decomposed ;  the  negative  fluid  flows  through  the  points  to  the  glass 
plate,  tending  to  neutralize  the  positive  fluid  on  the  plate.     The  con- 
ductors thus  lose  their  negative  electricity  and  become  charged  with 
positive  fluid. 

The  prime  conductor  does  not  acquire  positive  electricity  from  the 
plate,  but  gives  to  the  plate  its  negative  fluid,  thus  becoming  itself 
positive. 

If  a  metallic  point  be  held  at  some  distance  from  a  positively  charged 
prime  conductor,  the  electrometer,  F  (Fig.  15),  begins  to  fall,  showing 
a  loss  of  electricity.  But  the  point  does  not  draw  off  the  positive  elec- 
tricity from  the  conductor,  but  gives  to  the  conductor  negative  electri- 
city, which,  uniting  with  the  positive  fluid,  neutralizes  it. 

To  produce  negative  electricity,  the  machine  is  insulated  with  glass 
supports,  and  the  prime  conductor  connected  with  the  earth  by  a  me- 
tallic chain.  The  chain  permits  the  positive  fluid  to  escape  from  the 
prime  conductor,  whilst  the  negative  electricity,  being  unable  to  escape, 
accumulates  upon  the  cushions  and  frame  of  the  machine. 

680.  Measure  of  the  quantity  of  electricity  in  the  ma- 
chine.— The  degree  to  which  the  machine  is  charged  may  be  shown 
by  placing  a  quadrant  electrometer  (673)  upon  the  prime  conductor, 
as  seen  at  F,  Fig.  15.    When  the  machine  is  in  operation,  the  ball  rises 
along  the  quadrant,  and,  by  its  divergence  from  the  vertical  line,  indi- 
cates the  quantity  of  electricity  developed. 

Only  a  certain  amount  of  the  fluid  can  be  retained  on  the  prime 
conductor.  After  this  quantity  is  accumulated,  if  the  plate  is  turned, 


364  ELECTRICITY. 

the  tension  becomes  so  great,  it  escapes  through  the  air,  and  along  the 
glass  supports  of  the  machine. 

681.  Precautions  in  using-  the  machines. — A  dry  winter-air 
is  best  for  working  an  electrical  machine ;  and  an  apartment  heated 
by  dry  furnace-air  is  very  favorable. 

In  a  damp  day  electrical  experiments  are  seldom  performed  with 
success. 

In  carpeted  rooms  it  is  better  to  connect  the  rubbers  with  a  gas- 
fixture,  to  secure  a  good  communication  with  the  common  reservoir, 
the  earth.  The  machine  should  not  stand  near  the  walls  of  the  room, 
or  any  angular  body.  The  glass  columns  should  be  coated  with  an 
insulating  varnish  (674),  to  prevent  the  deposition  of  moisture. 

682.  Figure  16.— The  hydro-electric  machine.— For  fur- 
nishing electricity,  the  hydro-electric  machine  is  superior  to  any  above 

described.  This  is  an  apparatus  for  de- 
veloping electricity  from  high  steam.  It 
consists  of  an  insulated  steam-boiler, 
about  three  feet  long  and  twenty  inches 
diameter,  sufficiently  strong  to  sustain  a 
pressure  of  200  Ibs.  to  the  inch,  from 
which  comes  the  steam-pipe,  S,  in  the 
drawing.  T  is  a  box,  containing  a  little 
water,  through  which  the  steam  passes, 
in  its  passage  from  the  steam-pipe,  S,  to 
the  wooden  jets,  L.  In  these  jets  there  is 
a  sort  of  interrupted  passage  to  produce 
friction.  The  vapor  escapes  against  a 
number  of  metallic  points  in  the  frame 
above,  which  collect  the  electricity,  and 
communicate  with  the  insulated  brass 
ball,  H. 

The  evolution  of  electricity  is  due  to 
friction  between  the  particles  of  water 
(not  steam)  and  the  sides  of  the  discharge 
apertures ;  the  water  being  supplied  by  a  small  quantity  in  the  box,  T, 
through  which  the  steam  must  pass,  thereby  becoming  partially  con- 
densed. Steam  is  merely  the  vehicle  and  power  by  which  the  vesicles 
of  water  are  expelled. 

The  boiler  is  negative,  and  positive  electricity  is  collected  at  H,  as 
indicated  by  the  signs  plus  and  minus. 

Such  an  apparatus  will  develop,  in  a  given  time,  as  much  electricity 


ELECTRICITY.  365 

as  four  plate  machines,  forty  inches  in  diameter,  revolving  sixty  times 
a  minute. 

The  discovery  of  electricity  developed  by  steam  was  accidental.  An 
engineer,  endeavoring  to  cement  a  leak  in  a  steam-boiler,  observed  elec- 
trical phenomena,  which  subsequently  led  to  a  scientific  examination 
of  the  subject,  and  the  production  of  the  above  described  apparatus. 

688.  Other  sources  of  electrical  excitement. — 1.  Bands 
or  belts  of  leather,  India-rubber,  or  gutta-percha,  used  to  drive  ma- 
chinery, often  become  powerful  sources  of  resinous  electrical  excite- 
ment, giving  sparks  of  negative  electricity  twenty  or  more  inches  in 
length. 

2.  Negative  electrical  excitement  may  be  developed  by  briskly  rub- 
bing the  feet,  with  shoes  on,  around  011  a  woolen  carpet,  in  rooms 
heated  with  hot-air  furnaces;  the  person  performing  the  operation 
being  the  electrical  machine,  prime  conductor,  and  all.  A  person  thus 
electrized,  may  light  the  gas  by  a  spark  from  the  finger,  and  give  a 
shock  to  another  person.  This  experiment  is  most  successfully  per- 
formed when  the  wind  is  northwest. 


Experiments  Illustrating  Electrical  Attractions  and  Repulsions. 

684-  The  insulating  stool. — Electrical  spark.— Electrical 
shock. — Many  instructive  and  amusing  experiments  may  be  made 
with  an  electrical  machine,  illustrating  the  laws  of  electrical  attrac- 
tions and  repulsions.  Limited  space  will  admit  of  only  a  few. 

The  insulating  or  electrical  stool  is  nothing  more  than  a  simple 
bench,  provided  with  glass  feet  or  legs,  and  sufficiently  strong  to  sus- 
tain the  weight  of  a  person.  A  piece  of  board  resting  on  four  glass 
bottles  or  strong  tumblers  answers  every  purpose. 

An  electrical  spark  is  a  brilliant  flash  of  light  which  passes  when  a 
conductor  approaches  a  highly  electrified  body.  For  example,  if  the 
finger,  which  is  a  conductor,  is  held  near  to  a  charged  prime  conductor 
of  an  electrical  machine,  the  positive  fluid,  acting  at  a  distance  by  induc- 
tion, drives  the  positive  fluid  of  the  hand  to  the  earth,  and  the  body  of 
the  experimenter  becomes  negatively  electrified.  When  the  tension  of 
the  positive  fluid  of  the  machine  and  the  negative  fluid  of  the  body 
overcome  the  resistance  of  the  air,  they  rush  together  with  a  sharp 
crack  and  a  bright  light,  which  constitutes  the  spark.  If  the  electrical 
machine  is  sufficiently  powerful,  the  sparks  take  a  zig-zag  course,  like 
lightning,  shown  by  Fig.  26  (700). 

The  electrical  shock  is  the  sensation  experienced  by  the  person  who 
receives  the  electrical  spark. 


366 


ELECTRICITY. 


685.  Figure  17. — Electrical  puppets  consist  of  little  figures, 
made  of  cork,  pith,  or  some  similar  substance,  which  are  made  to  dance 

FIG.  17.  by  means  of  electrical  attraction  and 

repulsion.  These  are  placed  between 
two  metallic  plates,  as  shown ;  the 
lower  one  being  connected  with  the 
earth,  and  the  upper  one  suspended 
to  an  arm^of  the  prime  conductor. 

When  the  machine  is  turned,  the 
upper  plate,  becoming  electrified,  at- 
tracts the  images  Jx>  it;  these,  be- 
coming charged  with  positive  fluid, 
are  immediately  repelled  to  the  lower 
plate,  where  they  lose  this  fluid,  and 
are  again  attracted  to  the  upper 
plate ;  and  so  on,  dancing  up  and 
down  as  long  as  the  machine  is  turned. 
This  is  but  the  same  operation  illus- 
trated, in  a  simple  way,  by  Fig.  1 
(651). 

686.  Figure  18.— The  electrical  chime  signifies  the  ringing 
of  bells  by  means  of  electrical  attractions  and  repulsions. 

Attached  to  an  arm  of  the  prime  conductor  is  a  horizontal  'bar,  from 
FIG   18.  which  is  suspended  three  bells  and 

two  metallic  balls,  as  shown  in  the 
figure.  The  outer  bells,  E  and  F, 
are  suspended  by  metallic  chains  or 
wires,  and  the  middle  one,  L,  by  a 
non-conducting  substance,  as  a  silk 
cord;  the  middle  bell  is  also  con- 
nected with  the  earth  by  a  metallic 
chain. 

By  turning  the  machine,  the  bells, 
E  and  F,  becoming  positively  elec- 
trified by  the  prime  conductor,  at- 
tract the  balls,  which,  becoming 
positively  electrified,  are  immediately 
repelled;  and,  striking  against  the 
middle  bell,  L,  they  part  with  their 
charge,  and  are  again  attracted  by 
the  outer  bells  and  again  repelled. 

This  alternate  attraction  and  repulsion  keeps  the  bells  ringing  as  long 

as  the  plate  is  turned. 


ELECTRICITY. 


367 


FIG.  20. 


FIG.  21. 


687.  Figure  19.— The  electrical  wheel  consists  of  four  or 
more  pointed  arms,  bent  at  right  angles  at  the  ends,  and  connected  at 
the  centre  to  a  small  cap,  which  is  free  to  FIGK 

rotate  on  a  pivot  in  a  horizontal  plane. 

The  wheel  is  sustained  by  a  metallic 
support,  set  upon  an  arm  of  the  prime  con- 
ductor, as  shown  in  the  figure. 

When  the  machine  is  in  operation,  the 
wheel  becomes  electrified,  and  revolves. 
The  tension  of  the  electricity  at  the  points 
causes  the  fluid  to  escape,  which  causes  the 
wheel  to  revolve  by  the  reaction  of  the 
electricity  on  the  air.  The  wheel  will  not  revolve  in  a  vacuum,  though 
the  fluid  escapes  the  same ;  which  shows  that,  out  of  the  vacuum,  its 
motion  is  due  to  the  resistance  of  the  air  which  causes  the  reaction. 

688.  Figure  20.— The  electrical  blow-pipe.— If  a  pointed 
metallic  rod,  with  the  point  standing  horizontal,  be  placed  on  the 

prime  conductor,  L,  and 

the   electrical    machine 

put  in  operation,  the  es- 
cape of  electricity  from 

the  point  will  create  a 

current  of  air,  which  is 

rendered  sensible  to  the 

eye  by  placing  the  flame 

of  a  burning  candle  before  the  point,  as  represented 
in  the  figure.  The  current  from  an  active  machine 
will  even  extinguish  the  flame. 

689.  Figure  21. — The  electrical  egg  is 

an  egg-shaped  light,  produced  by  a  flow  of  electri- 
city through  a  vacuum. 

The  apparatus  for  exhibiting  this  light  consists 
of  a  hollow  globe  or  oval  of  glass,  containing  two 
small  spheres  of  metal,  F  and  H,  at  some  distance 
apart.  The  upper  one  is  connected  with  the  prime 
conductor  by  the  rod,  T,  and  the  lower  one  communicates  with  the 
earth.  At  the  bottom  of  the  glass  is  attached  a  pipe  (provided  with  a 
stop-cock,  N)  by  which  the  globe  is  attached  to  the  air-pump. 

Having  exhausted  the  air  and  closed  the  stop-cock,  if  the  machine 
be  turned,  a  flow  of  electricity  will  take  place  from  the  prime  conductor 
to  the  earth.  In  passing  from  the  ball  F  to  the  ball  H,  there  is  no  ob- 


368  ELECTRICITY. 

struction.  If  the  experiment  is  made  in  the  dark,  a  beautiful  violet- 
colored  light,  of  an  oval  form,  will  be  seen  between  the  balls,  as  shown 
in  the  figure. 

The  rarer  the  air  within  the  glass,  the  more  globular  will  be  the 
form  of  the  light. 


ACCUMULATION     OP     ELECTRICITY. 

690.  Latent  or  disguised  electricity. — When  two  equal  and 
insulated  conductors,  as  metallic  plates,  are  separated  from  each  other  by 
only  a  thin  plate  of  glass,  or  other   dielectric  substance,  and  then 
equally  excited  by  the  two  opposite  electricities,  no  evidences  whatever 
of  any  electrical  excitement  are  communicated  by  either  plate  to  an 
electroscope  connected  with  them.     The  glass,  or  dielectric,  prevents 
the  union  of  the  opposite  fluids,  but  does  not  prevent  their  inductive 
action  (670),  whereby  their  presence  is  entirely  masked  to  surrounding 
bodies. 

By  removing  the  plates  to  some  distance  from  each  other,  the  free 
electricity  of  each  becomes  manifest  by  its  effect  on  the  electroscope. 
If  they  be  again  brought  together,  with  the  dielectric  still  between 
them,  this  evidence  of  excitement  again  disappears;  and  so  on,  until, 
finally,  the  imperfect  insulation  of  the  air  allows  the  free  electricity  to 
become  neutralized. 

The  two  fluids  thus  situated  are  said  to  be  latent  or  disguised,  or 
paralyzed  by  their  mutual  attraction. 

691.  The  electrical  condenser. — An  electrical  condenser  is 
an  apparatus  employed  for  the  accumulation  of  electricity. 

Condensers  are  various  in  form,  but  in  principle  they  are  all  essen- 
tially the  same,  being  composed  of  two  conductors  separated  by  an  in- 
sulator, and  depend  upon  the  principle  above  explained  (690),  of  para- 
lyzing the  two  fluids,  or  rendering  them  latent. 

The  Leyden  jar  is  the  most  common  of  all  the  condensers. 

692.  Figure  22.— The  Leyden  jar.— This  valuable  piece  of 
apparatus,  named  from  the  city  where  it  was  invented,  was  first  dis- 
covered by  accident,  long  before  its  principle  of  action  was  under- 
stood. 

In  its  improved  form  it  consists  of  a  bottle  or  jar  of  thin  glass,  coated 
nearly  to  the  top,  on  the  inside  and  outside,  with  tin-foil ;  or,  instead 
of  coating  it  on  the  inside,  it  is  better  to  fill  it  nearly  full  of  loose  tin- 
foil, or  some  other  loose  metallic  substance,  as  shown  in  the  figure. 
A  metallic  rod,  passing  through  the  cork,  or  non-conducting  cover, 


ELECTRICITY.  369 

reaches  into  the  metallic  filling  ;  or,  if  FIG.  22. 

the  jar  be  coated  inside,  it  terminates 

with  a  metallic  chain,  the  lower  end  of 

which  falls  on  the  bottom  of  the  jar. 

The  rod  terminates  externally  with  a 

small  sphere  of  metal,  R,  called  the 

button. 

This  is  a  condenser  in  which  the 
glass  of  the  jar  serves  as  the  insulator 
or  dielectric  medium,  whilst  the  me- 
tallic substances,  within  and  without, 
correspond  to  the  two  plates,  previously 
mentioned  (690). 

The  discharger,  N,  will  be  explained 
hereafter. 

69 3.  Figure  23.— Charging  the  Leyden  jar.— The  Leyden 
jar  is  charged  by  holding  it  in  the  hand  (placing  the  hand  on  the 
tinned  part)  as  shown,  and  FlG 

bringing  the  button  in  contact 
with  the  prime  conductor,  F, 
of  the  electrical  machine. 

The  positive  electricity  is 
accumulated  in  the  interior, 
and  acts  by  induction  upon  the 
outer  coating,  which  becomes 
therefore  negative,  the  positive 
fluid  of  the  outer  coating  being 
conveyed  by  the  hand  through 
the  body  to  the  earth.  The  two 
fluids,  reacting,  accumulate  a 
large  quantity  of  positive  electricity  on  the  inside  of  the  jar,  and  nega- 
tive on  the  outside. 


694.  Limit  of  the  charge  in  a  condenser.— Disruptive 
discharge. — The  amount  of  electricity  that  can  be  accumulated  in  a 
condenser  is  limited  in  two  ways : 

1st.  By  the  limit  of  the  tension  of  the  fluid  in  the  charging  prime 
conductor ;  for,  when  the  tension  of  the  positive  fluid  on  its  plate  or 
inner  coating  of  the  Leyden  jar,  becomes  equal  to  that  on  the  prime 
conductor,  the  fluid  ceases  to  flow. 

2d.  By  the  cohesion  of  the  insulating  glass  or  dielectric  medium 
between  the  two  plates  or  coatings  of  the  jar;  for,  if  the  electrical 

24 


370  ELECTRICITY. 

machine  be  sufficiently  powerful,  the  tension  of  the  opposing  fluids 
goes  on  increasing  until  it  overcomes  the  cohesion  of  the  glass,  shiver- 
ing it  in  pieces,  producing  a  loud  explosion,  and  a  brilliant  spark. 
Such  a  case  as  this  is  called  a  disruptive  discharge. 

695.  The  discharge  of  the  condenser  or  Ley  den  jar  may 

take  place  in  four  ways : 

1st.  By  disruptive  discharge,  just  explained  (694). 

2d.  Insensibly  and  gradually,  by  imperfect  insulation,  especially  if 
the  air  be  damp. 

3d.  By  small  successive  discharges.  If  the  negative  plate  of  a  con- 
denser or  the  outer  coating  of  the  Leyden  jar  is  touched,  no  electricity 
is  drawn  off,  because  all  that  it  contains  is  held  in  equilibrium  by  the 
positive  fluid  of  the  other  plate  or  inner  coating  of  the  jar.  But  if  the 
positive  or  inner  coating  of  the  jar  is  touched,  all  of  its  free  electricity 
is  drawn  off,  that  is,  all  which  is  not  neutralized  by  the  other  plate  or 
coating  of  the  jar.  After  this  there  will  exist  on  the  negative  plate  or 
coating  of  the  jar  a  certain  portion  of  unneutralized  fluid,  indicated  by 
the  pith-ball  pendulum. 

By  continuing  to  touch  the  plates  alternately,  the  whole  charge  may 
be  drawn  off  in  small  quantities.  By  this  process,  whenever  either 
plate  or  coating  parts  with  its  free  electricity,  an  equal  quantity  of 
electricity  is  set  free  on  the  opposite  plate  or  coating. 

4th.  To  obtain  an  instantaneous  discharge,  it  is  only  necessary  to  put 
the  two  plates,  or  the  two  coatings  of  the  jar,  in  communication  with 
each  other,  by  means  of  a  conductor.  This  can  be  done  by  touching 
one  plate  with  the  right  hand  and  the  other  with  the  left;  or,  in 
case  of  the  Leyden  jar,  by  holding  the  jar  in  one  hand  and  touching 
the  button  with  the  other;  the  arms  and  body  being  the  conductor. 
The  fluids  flow  through  the  body  and  neutralize  each  other.  This 
produces  a  shock  far  more  powerful  than  that  produced  by  the  simple 
spark  from  the  prime  conductor.  To  avoid  the  shock  and  produce  the 
spark  and  explosion,  without  destroying  the  dielectric  (694),  an  instru- 
ment is  employed,  called  a  discharger. 

Dischargers. 

696.  The  discharging-rod  or  hand  discharger  (Fig.  22) 
consists  of  a  metallic  rod,  terminated  at  its  two  ends  by  small  balls  of 
metal,  /  and  g,  and  having  a  hinge-joint  at  the  middle,  N,  so  it  can  be 
folded,  to  vary  the  distance  between  the  balls.    It  is  also  provided  with 
an  insulating  handle. 

To  discharge  the  Leyden  jar  with  this  instrument,  it  is  only  neces- 


ELECTRICITY.  371 

sary  to  place  one  ball  in  contact  with  the  outer  coating,  S,  and  the 
other  with  the  button,  R. 

697.  Figure  24.— The  universal  discharger.— Of  the  vari- 
ous contrivances  for  regulating  the  discharge  of  condensers  and  the 
electric  battery,  the  universal  discharger,  represented  by  the  figure,  is 
considered  the  most  useful. 

FIG.  24. 


The  two  conducting-rods,  sliding  in  the  joints  E  and  F,  and  provided 
with  connecting  chains,  are  mounted  on  insulating  supports,  N  and  N; 
by  which  the  electrical  fluid  may  be  made  to  pass  through  any  sub- 
stance placed  upon  the  table,  H.  The  table  may  be  elevated  or  low- 
ered by  means  of  the  thumb-screw  on  the  left.  The  substance  exper- 
imented upon  is  insulated  by  a  plate  of  glass,  set  into  the  top  of  the 
table. 

The  conducting-rods  are  pointed  at  the  ends,  and  the  points  can  be 
exposed  by  unscrewing  the  balls  //.  The  rod  F  connects  with  the 
positive  side  of  the  battery,  while  the  rod  E  is  brought  into  contact 
with  the  negative  side,  by  means  of  the  chains,  or  by  applying  one  end 
of  the  discharging-rod  (696)  to  the  rod,  E,  and  the  other  end  to  the 
negative  side  of  the  battery. 

698.  Electricity  in  the  Leyden  jar  resides  on  the  glass. 

— This  is  shown  by  an  apparatus  or  jar  consisting  of  three  vessels 
(shaped  like  tapering  tumblers),  placed  inside  of  one  another.     The 
outer  and  inner  ones  being  thin  metallic  coatings,  and  the  middle 
one,  glass. 
If  this  jar  be  charged,  and  placed  on  an  insulating  surface,  and  the 


372  ELECTRICITY. 

vessels  separated,  the  electrometer  is  not  disturbed  by  either  the  outer 
or  inner  vessel,  while  the  glass  vessel  remains  strongly  excited.  If  the 
parts  be  again  put  together,  the  jar  is  found  to  be  charged  as  at  first. 

699.  Figure  25.— The  electric  battery.— An  electrical  bat- 
tery consists  of  several  Leyden  jars  connected  together  in  such  a  man- 
ner as  to  act  as  one  jar.  To 
establish  a  connection  between 
their  outer  coatings,  the  jars 
are  placed  in  a  box  which  is 
lined,  at  the  bottom,  with  thin 
metal.  Their  inside  surfaces 
are  brought  into  communica- 
tion by  connecting  the  several 
buttons  with  metallic  rods,  as 
represented  in  the  figure. 

Though  the  power  of  jars, 
other  things  being  equal,  is 
directly  as  their  surface,  yet  a 
limit  of  size  is  soon  reached, 
which  it  is  unprofitable  to  exceed,  owing  to  the  necessary  thickness  of 
glass,  etc.  Several  batteries  may  be  combined,  by  connecting  their  dis- 
charging-rods,  which  are  preferable  to  more  extended  single  series. 

A  battery  is  charged  same  as  a  single  jar ;  that  is,  by  connecting  the 
interior  with  the  prime  conductor,  and  the  exterior  with  the  earth. 
The  connection  is  made  with  the  prime  conductor  by  a  rod  passing 
from  one  of  the  buttons,  as  P ;  and  with  the  earth  by  a  chain  attached 
to  the  ring  in  the  handle  of  the  box — the  handle  being  in  metallic 
contact  with  the  lining  of  the  box. 

When  handling  powerful  batteries,  caution  is  requisite  to  avoid 
receiving  their  shocks,  else  serious  consequences  might  follow. 

A  battery  has  been  made  embracing  a  hundred  jars,  each  thirteen 
inches  in  diameter  and  two  feet  high.  Such  a  battery  magnetizes  steel, 
deflagrates  iron-wire,  dissipates  and  vaporizes  various  metals,  shivers 
blocks  of  wood  several  inches  square,  etc. 

.  Figure  26. — The  electric  spark. — The  explanation  of 
FIG.  26.  ^hig  figure  ig  contained  in  article 

684.  When  a  brass  ball,  at  the  end 
of  a  conducting-rod,  is  presented  to 
a  powerfully  charged  prime  con- 
ductor, sparks  are  sometimes  taken 
from  it  at  a  distance  of  thirty  inches. 


ELECTRICITY. 


373 


701.  The  color  of  the  electric  spark.— In  air  and  oxygen 
gas  it  is  white ;  though  in  heavy  thunder-storms  of  this  country  it  is 
sometimes  purple,  and  at  other  times  violet.  In  nitrogen  it  is  blue; 
in  hydrogen,  crimson  ;  in  carbonic  acid  gas,  green. 


FIG.  27. 


Fm 


702.  Figure  27.— Differ- 
ence between  the  positive 
and  negative  spark.— The  posi- 
tive electricity  gives  an  opening 
sheaf  or  brush  of  light;  negative 
electricity  gives  only  a  star,  as  rep- 
resented in  the  figure. 


703.  Figure  28.—  The  electrical  square.—  The  electrical 
square  consists  of  a  square  plate  of  glass,  upon  one  surface  of  which 
is  pasted  a  narrow  strip  of  tin-foil,  running  backward  and  forward 
across  the  plate,  as  shown  by  the  black  line  in  the  figure.  The  upper 
end  of  the  strip  is  connected  to  the  prime 
conductor  by  the  rod  P,  and  the  lower 
end  communicates  with  the  earth  by  the 
chain  T. 

If  the  strip  is  unbroken,  the  fluid 
passes  from  the  machine  to  the  earth  with- 
out emitting  sparks  ;  but  if  the  strip  be 
broken,  the  fluid,  in  passing  over  the 
break,  produces  a  continuous  light.  And 
if  several  breaks  be  made,  so  as  to  mark 
out  any  design,  as  letters  or  other  objects, 
the  design  will  appear  in  light,  as  if  traced 
on  the  glass  with  fire,  whenever  the  ma- 
chine is  turned.  The  experiment  is  more 
striking  in  a  dark  room. 


EFFECTS    OF    ACCUMULATED    ELECTRICITY. 

704.  The  effects  of  the  electric  discharge.— If  the  passage 
of  electricity  through  bodies  is  impeded  by  their  bad-conducting  qual- 
ity, or  by  want  of  proper  dimensions,  a  powerful  electric  discharge, 
under  such  circumstances,  will  produce  various  effects,  which  may  be 
classified  as  follows :  1st,  physiological ;  2d,  physical ;  3d,  mechanical ; 
4th,  chemical. 

705.  Physiological  effects  of  electricity  are  the  effects  which 
it  produces  on  men  and  animals.     They  consist  of  the  shock,  muscular 


374  ELECTRICITY. 

contractions,  more  or  less  pain,  and  death,  according  to  the  power  of 
the  electrical  apparatus. 

Any  number  of  persons,  by  joining  hands,  will  be  simultaneously 
and  similarly  affected  by  the  same  discharge.  An  electrical  shock  has 
been  administered,  in  this  manner,  to  five  hundred  persons  at  once. 

A  battery  of  six  jars,  of  average  size,  would  be  dangerous.  With 
more  powerful  batteries,  cats,  dogs,  and  larger  animals  may  be  killed 
outright.  There  are  batteries  of  sufficient  power  to  kill  an  ox 
instantly. 

A  person  charged  on  an  insulating  stool  (684),  feels  a  prickly  heat 
and  glow,  resulting  in  perspiration. 

Many  successful  applications  of  electricity  have  been  made  in  the 
treatment  and  cure  of  diseases. 

706.  Heating  power  of   electricity.— This  effect   of  elec- 
tricity is  shown  in  many  ways.    It  is  sufficiently  intense  not  only  to 
inflame  ether,  gunpowder,  etc.,  but   also  to  melt  and  volatilize  the 
metals  (699). 

The  heating  effect  is  shown  by  stretching  a  fine  wire,  L,  Fig.  24 
(697),  between  the  balls,//*,  of  the  universal  discharger,  and  discharging 
a  powerful  battery  through  it.  The  wire  will  undergo  combustion,  and 
be  dispersed  on  all  sides  with  vivid  scintillations. 

It  is  in  this  way  that  gunpowder  is  ignited  under  water,  for  blasting 
purposes,  blowing  up  ships,  etc. ;  the  wire  being  protected  from  the 
water  by  suitable  covering^ 

Though  no  heat  is  felt  when  the  knuckle  receives  strong  sparks  from 
an  active  machine,  yet  a  jet  of  burning  gas  can  be  inflamed  by  a  spark 
from  the  finger  (684)  ;  or,  more  strikingly,  from  an  icicle  held  in  the 
fingers  of  a  person  mounted  on  an  insulated  stool. 

707.  The  mechanical  effects  of  electricity  are  manifested 
when  powerful  charges  of  electricity  are  passed  through  imperfect  con- 
ductors.    The  effects  are  expansion,  with  tearing,  fracturing,  and  gen- 
eral shattering.     These  effects  are  exhibited  by  placing  the  body,  as  a 
billet  of  wood,  a  book,  or  a  box,  L,  Fig.  24  (697),  etc.,  on  the  table  of 
the  universal  discharger,  and  passing  through  them  a  powerful  charge 
from  the  battery.     In  this  way  blocks  of  wood  may  be  torn  to  splinters, 
holes  pierced  in  plates  of  glass,  and  through  books  of  four  or  five  hun- 
dred pages,  etc. 

70S.  The  chemical  effects  of  statical  electricity  are  gene- 
rally feeble.  Small  quantities  of  water  have  been  decomposed  witli 
very  small  submerged  points  of  gold.  Various  other  chemical  effects 


ELECTRICITY.  375 

have  been  observed,  but  these  belong  rather  to  the  subject  of  Chemistry 
than  to  this  branch  of  Physics  (see  752). 


ATMOSPHERIC     ELECTRICITY. 

9.  Franklin's  experiment  with  a  kite  proving  the 
identity  of  lightning  and  the  electrical  spark. — Franklin 
conceived  the  idea  that  the  phenomena  of  a  thunder-storm  were  due  to 
electricity,  and,  to  satisfy  his  inquiring  mind,  ingeniously  tested  the 
electrical  condition  of  the  clouds  with  a  kite,  and  with  such  success 
that  tears  accompanied  his  joy. 

Having  prepared  his  silken  kite,  with  a  pointed  wire  on  the  top,  and 
a  hempen  string,  at  the  lower  end  of  which  he  fastened  an  iron  key, 
and  to  the  key  an  insulating  silken  cord,  he  awaited  the  approach  of  a 
thunder-storm.  The  storm  came,  and  up  went  the  philosophic  and 
famous  kite,  which  soon  gave  the  great  philosopher  hopes  of  success ; 
and,  finally,  when  the  rain  had  increased  the  conducting  power  of  the 
string,  he  enjoyed  the  unspeakable  satisfaction  of  beholding  long  elec- 
trical sparks  darting  from  the  iron  key — which  unlocked  the  clouds  to 
his  searching  mind. 

Sparks  ten  feet  long  have  been  obtained  from  the  clouds  by  means 
of  kites. 

Vivid  sparks,  often  inconvenient  and  not  without  danger,  flow  from 
the  receiving  instruments  in  telegraph  offices  during  a  thunder- 
storm, the -wires  becoming  charged  with  atmospheric  electricity. 

710.  Free  electricity  in  the  atmosphere.— The  existence 
of  atmospheric  electricity  is  not  confined  to  clouds  alone,  for  it  often 
exists  in  the  atmosphere  when  no  trace  of  clouds  is  visible.  An  insu- 
lated conductor  extended  a  few  feet  into  the  air,  as  by  means  of  a  long 
fishing-rod,  will  affect  the  electrometer.  No  evidence,  however,  is  found 
of  the  existence  of  free  electricity  within  three  or  four  feet  of  the 
earth.  But  more  and  more  is  found  the  higher  the  conductor  is  raised, 
even  at  a  height  of  two  hundred  and  seventy-five  feet. 

From  many  experiments  it  is  found  that — 

1st.  The  electricity  of  the  atmosphere  is  always  positive;  is  most 
abundant  at  night ;  increases  after  sunrise ;  diminishes  toward  noon ; 
increases  again  toward  sunset. 

2d.  The  electrical  state  of  the  apparatus  is  disturbed  by  fogs,  rains, 
hail,  sleet,  or  snow ;  being  negative  when  these  approach ;  sometimes 
changing  from  positive  to  negative,  and  vice  versa,  every  three  or  four 
minutes. 

3d.  The  approach  of  clouds  affects  the  instrument  in  a  similar  manner. 


376  ELECTRICITY. 

4th.  Atmospheric  electricity  is  more  abundant  in  summer  than  in 
winter. 

711.  The  causes  of  atmospheric  electricity  are  :  1st.  The 
inductive  influence  of  the  negatively  excited  earth  ;  2d.  Evaporation; 
3d,  Condensation  ;  4th,  Vegetation  and  animal  life  ;  5th,  Combustion  ; 
6th,  Friction. 

The  first  of  these  causes  is  far  more  important  than  all  the  others. 
The  denser  air,  near  the  surface  of  the  earth,  acts  as  a  dielectric  be- 
tween the  negative  earth  and  positive  higher  layers  of  the  atmosphere; 
the  earth  and  air  being  an  immense  Leyden  jar. 


Thunder-storms  are  usually  attended  by  an  alteration  in 
the  direction  of  the  wind.  They  generally  prevail  in  the  lower  regions 
of  the  air,  and  are  most  frequent  and  violent  in  the  equatorial  regions, 
and  in  the  summer  than  in  the  winter.  They  are  attended  with  rapid 
condensation  of  atmospheric  vapor,  and  an  accumulation  of  electricity, 
in  which  they  chiefly  differ  from  other  storms. 

The  origin  of  thunder-clouds  is  due  to  the  rushing  up  of  the 
lighter  air  to  restore  the  normal  equilibrium  of  the  atmosphere,  which 
had  been  disturbed  by  the  gradual  introduction,  next  to  the  ground, 
of  warm  and  moist  air.  The  upper  end  of  such  an  ascending  column 
of  air  is  negatively  electrified,  as  its  lower  end  receives  positive  induc- 
tion from  the  negative  earth.  The  excess  of  watery  vapor  in  such  a 
cloud  will  be  precipitated  as  it  rises,  and  the  ascending  column  becomes 
a  conductor,  through  which  a  series  of  discharges  will  take  place  be- 
tween the  upper  and  lower  parts  of  the  cloud. 


Thunder  is  the  sound  which  follows  a  flash  of  lightning. 
The  lightning  passes  through  the  air  with  such  velocity,  it  violently 
displaces  it,  leaving  void  a  space  into  which  the  air  rushes  with  a  loud 
report. 

If  the  lightning  proceeds  from  or  toward  the  hearer,  the  sound  will 
be  somewhat  prolonged,  as,  in  this  case,  the  sound  from  different  parts 
of  the  vacuum  has  unequal  distances  to  travel  before  it  reaches  the  ear. 

The  loudness  of  thunder  depends  upon  its  nearness  and  the  power 
of  the  electric  discharge.  Near  by,  it  is  sharp  and  rattling  ;  at  a  greater 
distance  it  is  dull  and  prolonged. 

Lightning. 

714-  Lightning.  —  Air  subjected  to  compression  emits  a  spark; 
therefore  it  is,  by  some,  contended  that  lightning  is  due  to  condensa- 
tion of  air  in  front  of  the  electric  fluid,  in  its  rapid  progress  from 


ELECTRICITY.  377 

point  to  point.  At  any  rate,  it  is  the  result  of  electrical  discharges. 
Clouds  collect  and  retain  electricity ;  and  when  different  clouds  are 
unequally  or  differently  charged,  and  approach  each  other,  the  fluid 
rushes  from  one  to  another  through  the  intervening  air.  In  the  same 
manner  the  fluid  may  pass  from  the  clouds  to  the  earth,  when  the  dis- 
charge is  called  a  thunderbolt.  In  such  cases,  elevated  objects,  as  trees, 
church  steeples,  etc.,  often  govern  its  direction,  and  suffer  terrible 
consequences. 

In  low  regions  of  the  atmosphere  lightning  is  white ;  in  higher  re- 
gions it  is  violet. 

715.  Classes  of  lightning.— Lightning  has  been  divided  into 
classes ;  namely,  zig-zag  or  chain  lightning,  sheet  lightning,  ball  light- 
ning, heat  lightning,  and  volcanic  lightning. 

Zig-zag  or  chain  lightning. — The  zig-zag  form  is  due  to  the  fact  that 
the  compressed  air  in  front  of  the  fluid  resists  its  flow,  causing  it  to  be 
turned  aside  from  a  direct  course.  Sometimes  the  flash  is  thus  divided 
into  two  or  three  branches;  when  it  is  termed  forked  lightning. 

Sheet  lightning  is  a  diffused  glow  of  light  illuminating  the  borders 
of  the  clouds. 

Ball  lightning  appears  in  the  form  of  globular  masses,  sometimes 
remaining  stationary,  often  moving  slowly,  and  in  a  little  time  they 
explode  with  great  violence. 

Heat  lightning  occurs  often  in  serene  weather  near  the  horizon,  un- 
attended with  thunder.  It  is  the  reflection  in  the  atmosphere  of  light- 
ning at  a  remote  distance. 

Volcanic  lightning  is  caused  by  rapid  condensation  of  the  vast  volumes 
of  heated  vapor,  thrown  into  the  air  from  active  volcanoes.  This  class 
of  lightning  is  sometimes  very  terrific. 

716.  The  velocity  of  lightning  is  estimated  to  be  not  less  than 
250,000  miles  per  second.     The  duration  of  a  flash  of  lightning  does 
not  exceed  a  millionth  part  of  a  second. 

7 17.  The  return-shock  is  a  violent  shock  felt  at  a  great  distance 
from  the  place  where  the  lightning  strikes.    It  is  due  to  the  induction 
of  an  electrified  cloud  upon  the  ground  and  bodies  beneath  it,  which 
are  all  strongly  charged  with  electricity  contrary  to  that  of  the  cloud. 
When  a  discharge  takes  place,  at  whatever  point  of  the  cloud,  the  cloud 
returns  to  its  neutral  state,  causing  its  inductive  influence  to  cease 
instantly,  whereupon  all  the  bodies  electrified  by  its  induction  instantly 
return  to  the  neutral  state.     The  violence  of  this  return  constitutes  the 
return-shock,  which  sometimes  causes  death. 


378 


ELECTRICITY. 


FIG.  29. 


7 18.  Figure  29. — Lightning-rods  are  rods  of  metal  attached 
to  buildings  and  ships,  to  protect  them  from  injurious  effects  of  lightning. 
The  protective  influence  of  a  rod  extends  to  a  dis- 
tance from  itself  equal  to  four  times  its  height  above 
the  building. 

They  receive  the  fluid  on  the  point,  and  silently 
convey  it  from  the  clouds  to  the  ground,  even  when 
no  visible  discharge  takes  place. 

To  render  lightning-rods  effective,  they  should  be 
well  insulated  from  the  building  by  glass  holders, 
pointed  at  the  top  and  the  point  covered  with  non- 
corrosive  metal,  of  ample  size  and  good  conducting 
material  (copper  is  best),  set  deep  enough  in  the 
ground  to  reach  the  permanently  moist  earth,  con- 
nected with  other  metallic  substances  (if  any)  on 
the  building,  as  gutters,  gas  pipes,  etc. ;  and  espe- 
cially should  they  be  continuous,  otherwise  they  are 
a  source  of  danger  rather  than  safety,  as  will  be 
seen  by  the  following  experiment. 

The  little  tower  (Fig.  29),  constructed  of  separate  wooden  blocks, 
rests  on  three  metallic  balls  or  buttons,  placed  on  a  base  block,  as  rep- 
resented in  the  figure.  The  button  in  front  stands  upon  a  small  square 
piece,  shown  separately  at  E.  This  piece  has  a  metallic  connection 
running  through  it  in  one  direction,  and  half  way  through  it  in  an- 
other ;  so  that,  by  inserting  this  piece  into  its  recess  one  side  up,  the 
lightning-rod  (represented  by  the  dotted  line)  is  made  continuous, 
from  the  top  of  the  tower  to  the  chain  below.  By  turning  the  piece 
over,  the  metallic  connection  can  be  broken.  If  the  charge  of  a  Leyden 
jar  is  passed  through  the  rod  when  the  circuit  is  interrupted,  the  piece 
will  be  blown  out,  and  the  tower  thrown  down.  By  turning  the  block, 
and  thus  completing  the  metallic  connection,  the  same  charge  is  passed 
without  disturbing  the  structure. 

Means  of  safety. — Persons  who  suffer  with  fear  of  being  struck 
by  lightning,  may  feel  a  sense  of  security  by  putting  on  silk  clothing, 
and  sitting  in  an  insulated  chair  placed  in  the  middle  of  the  room. 
The  chair  can  be  conveniently  insulated  with  four  glass  tumblers,  or 
pieces  of  thick  glass.  Or  they  may  place  themselves  upon  a  feather 
bed.  It  is  safer  to  avoid  currents  of  air,  and  nearness  to  chimneys,  and 
walls  of  the  room,  and  metal  conductors,  as  gilt  frames,  etc.  If  out  of 
doors,  it  is  prudent  to  avoid  elevated  objects,  as  trees,  buildings, 
etc.  For  obvious  reasons,  it  is  safer  to  be  in  valleys  than  on  hill-tops 
or  hill-sides. 


ELECTRICITY.  379 


Liability  of  being  struck  by  lightning.  —  The  apprehension 
and  solicitude  respecting  lightning  are  proportionate  to  the  magnitude 
of  the  evils  it  produces,  rather  than  the  frequency  of  its  occurrence. 
The  chances  of  our  being  killed  or  injured  by  lightning  are  infinitely 
less  than  those  which  we  encounter  in  travelling  by  boats  and  cars,  or 
in  our  daily  walks  and  occupations,  or  even  in  our  sleep  from  the  des- 
truction of  our  dwellings  by  fire. 


.  Aurora  borealis.  —  By  this  term  is  meant  the  luminous 
phenomena  which  are  often  seen  in  the  regions  of  the  poles  of  the 
earth.  In  the  northern  hemisphere  they  appear  in  the  north  ;  in  the 
southern  hemisphere  they  appear  in  the  south,  and  are  then  called 
aurora  australis.  There  are  different  opinions  concerning  the  cause 
of  these  phenomena.  By  some  they  are  supposed  to  be  due  to  the  pas- 
sage of  electric  currents  through  the  higher  regions  of  the  atmosphere; 
the  different  colors  being  manifested  by  the  passage  of  the  fluid  through 
air  of  different  densities.  Though  philosophers  cannot  yet  demonstrate 
the  cause  of  these  luminous  appearances,  still  it  is  generally  believed 
that  the  phenomena  are  intimately  connected  with  terrestrial  magnetic 
electricity. 

That  this  light  is  not  a  local  phenomenon  is  evident,  for  the  reason 
that  it  is  often  seen  simultaneously  in  places  far  apart,  as  Europe  and 
America. 

The  height  of  the  auroras  has  been  estimated  to  be  from  one  hundred 
to  two  hundred  miles. 

They  appear  to  be  more  frequent  about  the  period  of  the  equinoxes 
than  at  other  times. 

During  the  prevalence  of  the  auroras,  all  the  magnetic  elements 
show  great  disturbance,  simultaneously,  at  the  most  distant  sta- 
tions. 

That  the  auroras  act  upon  telegraphic  wires,  is  shown  by  the  fact 
that  several  telegraphic  lines  in  the  United  States  were  worked,  in 
parts  of  August  and  September  of  1859,  for  hours  together,  entirely  by 
the  magnetic  current  induced  by  the  aurora,  the  batteries  being  de- 
tached. 

Chemical  decomposition,  and  heating  and  luminous  effects,  have  been 
observed  from  the  currents  induced  during  auroral  disturbances. 

As  everybody  has  observed,  or  may  observe,  the  auroras,  often  called 
the  Northern  Lights,  it  is  useless  to  describe  their  general  appear- 
ance. 

The  phenomena  of  the  auroras  occur  in  the  day-time  as  well  as  at 
night,  but  the  superior  light  of  the  sun  renders  the  auroral  light  invis- 
ible during  the  day. 


380 


ELECTRICITY. 


FIG.  30. 


Figure  30.— Slow  discharge  of  a  Leyden  jar.— 

Before  leaving  the  topic  of  statical  or  frictional  electricity,  and  taking 

up  that  branch  of  the  general  subject 
termed  dynamical  electricity,  we  will  describe 
what  may  be  considered  a  beautiful  elec- 
trical toy,  consisting  essentially  of  a  Leyden 
jar  and  two  small  bells. 

A  charged  jar,  provided  with  a  small  bell 
in  place  of  the  button  or  knob,  is  set  upon  a 
board,  near  to  a  small  brass  ball  fastened  to 
a  silken  thread,  which  is  held  at  the  upper 
end  by  a  metallic  rod,  A.  This  rod  connects 
with  the  earth,  and  supports  another  small 
bell. 

The  positive  electricity  of  the  jar  attracts 
the  little  ball,  which,  after  striking  the  bell, 
is  repelled,  until  it  strikes  the  other  bell, 
when  its  positive  fluid  is  discharged,  and 
itself  is  again  attracted  by  the  first  bell,  and 
so  on,  for  many  hours  (653). 


CHAPTEK   XV. 

DYNAMICAL   ELECTRICITY. 
FUNDAMENTAL     PKINCIPLES. 

Galvanism. — Electricity  excited  or  produced  by  chemical 
action  is  called  Galvanism,  in  honor  of  Galvani,  who  first  observed 
certain  phenomena  which  led  to  the  discovery  of  generating  electricity 
in  this  way. 


Figure  31. — Galvani's  discovery  and  experiments. 

— In  the  year  1786,  Luigi  Galvani,  professor  of  anatomy  in  the  Uni- 
versity of  Bologna,  while  making  applications  of  atmospheric  electri- 
city to  animal  organisms,  accidentally  observed  convulsive  movements 
in  the  body  of  a  frog,  which  he  had  prepared  for  some  experiment,  and 
hung  on  a  copper  hook  near  an  iron  frame  of  the  window.  By 
further  observation  and  experiment,  he  found  that  the  convulsions 
were  strongest  when  he  made  connection,  by  means  of  two  metals,  be- 
tween the  lumbar  nerves  and  the  exterior  muscles,  denuded  of  the  skin, 
as  represented  in  the  figure. 


ELECTRICITY. 


381 


The  instrument  in  the  hand  FIG.  31. 

consists  of  two  pieces  of  metal, 
zinc,  Z,  and  copper,  C,  joined 
together  at  the  contact  with 
the  handle.  By  simply  placing 
the  zinc  in  contact  with  the 
nerves,  at  N,  no  convulsion  oc- 
curs, but  as  soon  as  the  copper 
conies  in  contact  with  the  limb 
below,  the  leg  is  convulsed  and 
drawn  up,  as  shown. 

Galvani  found  also  that  these 
convulsions  could  be  produced 
by  bringing  the  interior  surface 
of  the  nerves  in  contact  with  the 
exterior  mucous  surface,  without 
the  use  of  metals. 

Thus  began  the  discovery  of 
the  method  of  generating  electricity  by  means  of  chemical  action,  now 
constituting  one  of  the  most  important  branches  of  science. 

723 .  Galvani' s  explanation.— Galvani  held  that  the  convul- 
sions of  the  frog  were  excited  by  a  nervous  or  vital  fluid,  passing  from 
the  nerves  to  the  muscles  by  way  of  the  metallic  communication,  and 
that,  falling  on  the  muscles,  it  contracted  them,  like  the  electrical  dis- 
charge from  the  machine.  This  view  was  soon  shown  to  be  incorrect, 
but  was  adhered  to  by  Galvani  until  he  died,  in  1798,  before  the  Voltaic 
Pile  was  given  to  the  world. 

7% 4-  Volte's  theory  of  contact.— Volta  at  first  accepted  the 
views  of  Galvani,  but  after  experiment  and  study,  gave  up  this  hypo- 
thesis and  adopted  one  of  his  own.  He  attributed  the  electrical  effects, 
manifested  by  the  convulsions,  to  the  contact  of  dissimilar  substances, 
which,  he  claimed,  caused  a  decomposition  of  the  natural  electricity  of 
both  bodies,  the  positive  fluid  going  to  one  and  the  negative  to  the 
other;  and  that  the  frog's  limbs  were  only  the  sensitive  electroscope, 
indicating  the  current  thus  developed. 

Though  this  theory  held  general  sway  for  a  long  time,  yet  it  was  not 
the  true  explanation ;  hence  it  was  gradually  supplanted  by  the  electro- 
chemical theory,  which  refers  these  phenomena  to  chemical  action. 


Volta  discovered  that  certain  metals,  particularly  the  oxydizable 
metals,  disengage  electricity  and  charge  the  condenser. 


382  ELECTRICITY. 

' 

This  discovery  soon  led  (1800)  to  Volta's  second  and  great  discovery, 
viz.,  the  Voltaic  pile  or  battery. 

725.  The  electro-chemical  theory. — The  true  cause  of  elec- 
trical excitement,  occasioned   by  the  contact  of   dissimilar  metals,  is 
now  fully  ascertained  to  be  chemical  action. 

Electricity  produced  by  chemical  action  is  termed  Galvanic  or 
Voltaic  electricity.  Yet  Galvani  never  even  saw  a  galvanic  battery ; 
but,  having  been  the  first  observer  of  the  phenomena  which  set  Volta 
to  work,  he  shares  the  honor  with  Volta.  Yet  neither  of  them  fully 
understood  the  discovery,  which,  through  their  instrumentality,  is  now 
so  beneficial  to  mankind. 

Fabroni  first  suggested  the  chemical  theory,  the  truth  of  which  was 
subsequently  demonstrated  by  De  le  Rive  and  Becquerel,  who  also 
found  that  no  chemical  action  takes  place  without  developing  electricity. 

They  also  showed  that  whenever  a  metal  is  attacked  by  an  acid,  the 
former  is  positively  and  the  latter  negatively  electrified. 

726.  Figure     32. — Simple    voltaic    couple.  —  Whenever 
two  unlike  substances,  moistened  by  or  immersed  in  an  acid  or  saline 
fluid,  are  brought  into  contact,  a  voltaic  current  is  established. 

If  a  silver  or  copper  coin  and  a  piece  of  zinc  be  placed  one  above  and 
FIG.  32.  the  other  below  the  tongue,  so  that  their 

edges  are  in  contact,  a  prickly  sensation 
will  be  felt  in  the  tongue,  and,  if  the  eyes 
be  closed,  a  mild  flash  of  light  is  also  seen. 
A  mention  of  this  simple  experiment  by 
Sulzer,  a  German  author,  is  the  first  re- 
corded phenomenon  attributable  to  voltaic 
electricity.  The  saliva,  in  this  case,  is  the 
saline  fluid,  exciting  a  voltaic  current,  due 
to  its  chemical  eifect  on  the  zinc  or  copper, 
and  the  nerves  of  sense  are  the  electro- 
scope. 

The  figure  represents  the  simplest  form 
of  a  voltaic  battery.  It  consists  of  a  piece 
of  copper  (c)  and  a  piece  of  zinc  (2),  partly 
immersed  in  a  weak  solution  of  sulphuric 
acid,  resting  in  direct  contact  at  the  top, 
or  united  by  means  of  copper  wires,  as  shown  in  the  figure.  Thus,  it 
is  seen  that  a  simple  voltaic  couple  consists  of  three  elements — zinc, 
acid,  copper. 

"When  the  two  metals  are  in  contact  (within  or  without  the  liquid), 


ELECTRICITY. 


383 


two  electrical  currents  will  be  set  up,  flowing  in  opposite  directions,  as 
indicated  by  the  two  systems  of  arrows,  in  either  of  the  two  couples 
in  the  figure. 

The  positive  fluid  flows  from  the  zinc  to  the  copper  in  the  liquid  and 
from  the  copper  to  the  zinc  in  the  air,  as  indicated  by  the  long  arrows 
at  the  top  and  bottom ;  while  the  negative  fluid  passes  from  the  copper 
to  the  zinc  in  the  liquid  and  from  the  zinc  to  the  copper  in  the  air,  as 
indicated  by  the  short  arrows  at  the  top  and  bottom,  and  by  the  signs 
plus  and  minus. 

Evidence  of  chemical  action  is  seen  by  the  constant  flow  of  gas  bub- 
bles (hydrogen)  from  the  zinc  to  the  copper.  This  action  ceases  the 
instant  the  contact  between  the  metals  is  broken. 

727.  Figure  33.— The  voltaic  pile  or  battery.— In  the  year 
1800,  Volta  invented  an  apparatus  by  which  the  number  of  contacts 
could  be  multiplied,  and  the  effect  increased. 

This  he  did  in  accordance  with  his  contact  FIG.  33. 

theory,  previously  mentioned  (724). 

Such  a  pile  consisted  of  alternate  copper 
and  zinc  disks,  laid  up  as  shown  in  the  figure, 
with  disks  of  paper  or  cloth  between  them, 
moistened  with  brine  or  acidulated  water; 
the  couples  being  all  disposed  in  the  same 
order,  that  is,  copper,  cloth,  zinc, — copper, 
cloth,  zinc,  and  so  on.  W  stands  for  the  wet 
cloth  in  the  drawing. 

The  terminal  plates  (copper  at  bottom  and 
zinc  at  top)  are  provided  with  ears,  for  the 
convenient  attachment  of  wires.  The  wires 
connecting  the  terminal  plates  are  called 
electrodes. 

Each  couplet  of  copper  and  zinc  may  be 
soldered  together,  and  then  called  a  couple, 
pair,  or  voltaic  element. 

Other  metals  may  be  employed,  but  these  are  good,  convenient,  and 
cheap. 

728.  Varieties  of  voltaic  piles. — As  voltaic  piles  do  not  de- 
pend upon  any  particular  form  or  substances,  they  have  been  made  in 
various  forms  and  of  various  materials.     They  have  been  made  wholly 
of  animal  substances,  and  entirely  of  vegetable  substances,  as  disks  of 
beet-root  and  walnut-wood  in  contact;  it  being  only  required  that  the 
different  substances  act  chemically  upon  each  other. 


384  ELECTRICITY. 

A  perfectly  dry  pile  may  be  made  of  small  disks  of  gilded  paper  and 
sheet-zinc,  packed  in  a  glass  tube.  They  have  been  made  in  this  way, 
consisting  of  10,000  pairs,  yielding  sparks,  charging  Leyden  jars,  ring- 
ing bells,  etc.,  for  twenty  years. 

The  voltaic  or  galvanic  batteries,  in  use  at  the  present  day,  are  very 
different  in  form  and  efficiency  from  those  formerly  employed. 

The  trough  lattery  was  successful.  It  was  with  one  of  these  that 
Davy  made  his  invaluable  and  immortal  discovery  of  the  metallic  bases 
of  the  alkalies. 

729.  Polarity  of  the  pile. — The  pile  being  insulated  and  tested, 
is  found  to  possess  electrical  polarity, — one  half  of  the  pile  being  posi- 
tive, the  other  half  negative,  and  the  middle  neutral.     In  the  zinc  and 
copper  pile,  the  end  terminating  with  zinc  is  positive,  the  other  end 
being  negative,  as  indicated  by  the  signs  plus  and  minus,  in  Fig.  33. 

The  tension  is  greatest  at  the  extremities ;  hence  these  are  named 
poles.  In  any  case,  the  end  yielding  positive  fluid  is  the  positive  pole; 
the  end  yielding  negative  fluid  is  the  negative  pole. 

The  pile,  therefore,  may  be  regarded  as  a  Leyden  jar,  or  electrical 
battery,  perpetually  charged,  if  the  necessary  conditions  are  maintained. 

730.  Electrical  currents  of  the  pile. — The  pile  manifests  no 
electrical  action  as  long  as  the  electrodes  remain  separated,  but  if  these 
are  brought  near  each  other,  a  small  spark  will  pass,  which  is  caused 
by  a  re-combination  of  the  two  fluids.     The  passage  of  the  spark  does 
not  discharge  the  pile,  as  in  the  case  of  the  Leyden  jar;  but  a  succes- 
sion of  sparks  will  pass,  showing  that  the  process  of  decomposition  of 
electricity  in  the  pile  is  constantly  going  on,  by  which  its  poles  are 
continually  supplied  with  positive  and  negative  fluids. 

If  the  wires  or  electrodes  are  brought  into  actual  contact,  the  sparks 
cease,  but  the  flow  of  the  fluid  continues  the  same.  This  continuous 
flow  of  electricity  is  called  the  electrical  current. 

There  are  two  currents  of  electricity,  positive  and  negative  (726), 
flowing  in  opposite  directions.  For  convenience,  only  one  of  them  will 
be  considered,  namely,  that  which  flows  from  the  positive  to  the  nega- 
tive pole. 

731.  Electro-positive   and   electro-negative.  —  The   two 

metals,  when  placed  in  contact,  are  said  to  be  electro-polarized.  The 
one  giving  traces  of  free  positive  electricity  is  said  to  be  electro-positive, 
and  the  other  electro-negative.  In  case  of  zinc  and  copper,  the  zinc  is 
electro-positive  and  the  copper  electro-negative. 

These  are  only  relative  terms ;  as  the  same  metal  may  be  electro- 


ELECTRICITY.  385 

positive   when   coupled  with  one  metal,  and  electro-negative  when 
coupled  with  another. 

In  any  case,  the  electro-positive  metal  is  the  one  most  easily  corroded 
or  oxydized. 


The  difference  between  quantity  and  intensity.— 

The  electricity  evolved  by  a  single  voltaic  couple  is  considerable  in 
quantity,  but  weak  in  intensity.  Electricity  produced  by  the  machine 
is  small  in  quantity,  but  of  high  tension;  that  is,  capable  of  passing 
through  air,  but  producing  slight  chemical  or  heating  effects.  On  the 
contrary,  voltaic  electricity  is  great  in  quantity  and  small  in  intensity  ; 
shown  by  the  fact  that  the  thinnest  film  of  air  is  a  perfect  insulator,  — 
and  so  are  all  dry  woods,  —  but  its  chemical  and  heating  effects  are 
powerful. 

733.  Quantity  increases  with  surface,  intensity  with 
number  of  pairs.  —  The  quantity  of  electricity  increases  with  the 
surface,  but  not  with  the  number  of  the  pairs  ;  hence,  to  increase  the 
quantity,  large  plates  are  employed. 

The  tension  increases  with  the  number  of  the  pairs.  No  greater 
quantity  of  electricity  is  obtained  from  a  pile  than  a  single  pair  of 
plates,  its  intensity  alone  being  increased. 

7  SJf-  Amalgamated  zinc.  —  As  all  commercial  zinc  contains 
more  or  less  of  foreign  substances,  which  stand  in  the  electro-negative 
relation  to  the  zinc,  it  is  necessary  to  protect  them  from  the  action  of 
the  acid.  Otherwise,  each  particle  of  the  impurities  forms,  with  the 
contiguous  particle  of  zinc,  a  minute  battery  ;  which  rapidly  corrodes 
and  roughens  the  surface,  and  correspondingly  destroys  the  power  of 
the  whole  couplet.  This  action  of  common  zinc  is  called  a  local  ac- 
tion. 

To  prevent  this  action,  the  freshly  corroded  zinc  is  rubbed  with  a 
little  mercury,  which  covers  it  with  a  uniform  coating  of  zinc  amal- 
gam. Thus  prepared,  zinc  may  be  left  in  the  acid  water  without  in- 
jury, and  when  brought  into  contact  with  the  other  metal  of  the 
battery,  it  becomes  far  more  energetic  than  before.  This  zinc  amalgam 
is  indispensable  in  practice. 

Batteries.  „ 

735.  Smee's  battery  consists  of  a  plate  of  silver  coated  with 
platinum,  suspended  between  two  plates  of  amalgamated  zinc.  The  three 
are  attached  to  a  wooden  bar,  which  supports  the  whole  in  a  tumbler 
partly  filled  with  water,  acidulated  with  one-seventh  of  its  bulk  of  sul- 

25 


386 


ELECTRICITY. 


phuric  acid  (blue  vitriol) ;  or,  for  less  activity,  one-sixteenth.     The  elec- 
trodes are  fastened  to  the  zinc  and  silver  plates  by  means  of  small  screws. 
FlG  34  The  quantity,  but  not  the  inten- 

sity, of  this  battery,  is  very  great ; 
and  for  many  days  it  maintains  a 
uniform  action. 

736.  Figure  34.— The  sul- 
phate of  copper  battery  con- 
sists of  two  concentric  cylinders  of 
copper,  cc,  tightly  soldered  to  a 
copper  bottom,  and  a  zinc  cylinder, 
z,  fitting  between  them.  The  li- 
quid employed  is  a  solution  of  sul- 
phate of  copper  (blue  vitriol).  The 
zinc  cylinder  is  prevented  from 
touching  the  copper  by  means  of 
three  pieces  of  wood  or  ivory  (not 
shown)  projecting  from  the  top  of 
the  zinc  cylinder,  and  resting  on 
the  top  of  the  outer  copper  cylinder. 
The  electrodes  are  connected,  one 
with  the  outer  copper  cylinder,  and  the  other  with  the  zinc,  as  repre- 
sented. 

FIG.  35.  FlG-  36- 


737.  Figure  35.— Bohnenberger's  dry  pile  electroscope 

consists'  of  two  dry  voltaic  piles  (728),  arranged  under  a  glass  jar, 


ELECTRICITY.  387 


having  a  strip  of  gold-leaf  or  a  pith-ball  suspended  between  their  op- 
posite poles,  R  and  H,  and  connected  with  the  rod  and  knob,  P. 

If  a  positively-electrified  body,  S,  be  brought  near  to  the  knob,  P, 
the  gold-leaf  will  be  attracted  to  the  negative  pole,  H,  of  the  electro- 
scope ;  but  if  a  negatively-electrified  body  be  brought  near  to  the  knob, 
the  gold-leaf  will  be  attracted  to  the  positive  pole,  R.  This  is  one  of 
the  most  delicate  electroscopes. 

738.  Figure  36.— Grove's  nitric  acid  battery.— This  is  a 

powerful  and  intense  sustaining  battery.  The  outer  vessel  is  glass,  filled 
with  from  six  to  ten  parts  of  water  to  one  of  sulphuric  acid.  In  this  fluid 
is  placed  an  amalgamated  zinc  cylinder,  Z,  open  on  one  side,  as  shown. 
The  inner  vessel,  V,  is  a  porous  jar,  filled  with  nitric  acid,  in  which  is 
immersed  a  piece  of  platinum,  P.  The  porous  vessel  is  covered  to  keep 
down  the  fumes  of  the  acid.  The  connecting  wires  are  held  by  bind- 
ing screws,  shown  at  the  top. 

In  this  battery  there  is  a  double  chemical  action,  the  hydrogen 
being  engaged  by  the  nitric  acid,  which  it  readily  decomposes.  There 
is  therefore  an  increased  flow  of  electricity. 

739.  Figure   37.— Carbon   battery.— This  battery  is  essen- 
tially like  the  one  last  described,  with  the  exception  that  carbon  is 
substituted  for  the  platinum,  to  save  ex-  FIG.  37. 

pense. 

E  is  an  earthen  vessel,  containing  di- 
lute sulphuric  acid ;  Z,  a  zinc  cylinder, 
open  on  one  side,  having  a  strip  of  copper 
soldered  to  its  upper  edge  ;  V,  a  vessel  of 
porous  earthenware,  containing  nitric 
acid ;  c.  a  cylinder  of  well-calcined  car- 
bon or  coke,  which  is  a  good  conductor. 
In  the  top  of  this  cylinder  a  stem  of  cop- 
per is  inserted,  to  which  is  soldered  a  strip 
of  the  same  metal,  which,  with  the  other 
strip  of  copper,  constitutes  the  electrodes. 

As  in  the  Grove  battery  (738),  there  is 
a  double  chemical  action.  Water  is  de- 
composed in  the  outer  vessel,  giving  its 
oxygen  to  the  zinc,  forming  oxyde  of 
zinc.  The  liberated  hydrogen  passes  through  the  porous  vessel.  V, 
and,  uniting  with  a  part  of  the  oxygen  of  the  nitric  acid,  decomposes 
it,  producing  water,  and  also  forming  nitrous  acid,  which  escapes  in 
fumes.  This  double  action  developes  a  large  amount  of  electricity. 


388  ELECTRICITY. 

The  carbon  is  the  positive  and  the  zinc  the  negative  pole  of  the  couple. 
The  positive  fluid,  therefore,  passes  from  the  carbon  to  the  zinc. 

740.  Figure  38. — Batteries  of  two  or  more  couples.— 

Any  number  of  couples  may  be  united  by  attaching  the  copper  strip  of 
the  zinc  cylinder  in  one  couple  to  that  of  the  carbon  in  the  next 
couple,  and  so  on  throughout  the  combination.  The  remaining  two 
strips,  which  will  be  one  on  the  first  and  the  other  on  the  last  couple, 
may  be  united  by  a  conductor. 


FIG.  38. 


Such  a  combination  is  shown  by  the  figure,  consisting  of  sixteen 
carbon  batteries,  the  view  being  from  above,  looking  down  upon  the 
combination. 

The  outer  circle  of  each  couple  represents  the  outer  vessel  ;  the  next 
circle,  the  zinc  ;  the  next,  the  vessel  containing  the  acid  ;  and  the 
central  circle,  the  carbon. 

It  will  be  noticed  that  the  carbon  in  all  the  couples  is  marked  plus 
(  -h)j  showing  that  the  positive  fluid  passes  from  the  carbon  to  the 
zinc;  and  that  the  strips  are  marked  minus  (  —  ),  where  they  join  the 
zinc,  showing  that  the  negative  fluid  passes  from  the  zinc  to  the  carbon. 


Resistance  to  the  current.  —  The  electric  current  must 
proceed  not  only  along  the  connecting  wire,  from  pole  to  pole,  but  also 


electro-motive  force. — By  the  electro-motive  force 
is  meant  the  cause  which  gives  rise  to  the  electric  current,  which  is  the 
oxydation  of  the  zinc ;  in  other  words,  the  chemical  action. 


ELECTRICITY.  389 

through  the  couples ;  hence,  there  is  a  resistance  to  the  flow  of  the 
current  exterior  to  and  within  the  apparatus. 

743.  Laws  determining  the  force  of  a  voltaic  current. 

1st.  The  electro-motive  force  varies  with  the  number  of  the  elements 
or  pairs,  the  nature  of  the  metals,  and  the  nature  of  the  liquids  which 
constitute  the  elements ;  but  it  does  not,  in  any  manner,  depend  on 
the  dimensions  of  their  parts. 

2d.  The  resistance  of  each  element  or  pair  is  directly  proportional 
to  the  distance  between  the  plates  within  the  liquid,  the  resistance  of 
the  liquid  itself,  and  the  length  of  the  wire  completing  the  circuit ;  and 
inversely  proportional  to  the  surface  of  the  plates  in  contact  with  the 
liquid,  and  to  the  section  or  size  of  the  connecting  wire. 

3d.  The  force  of  the  current  is  equal  to  the  electro-motive  force, 
divided  by  the  resistance. 

7 44-  Difference  between  static  and  dynamic  electricity. 

— The  nature  of  electricity  is  the  same,  whether  it  be  produced  by  fric- 
tion, chemical  action,  or  other  means. 

The  difference  between  frictional  electricity  and  that  evolved  by 
chemical  action,  consists  in  the  low  tension  and  great  quantity  of  the 
latter,  as  compared  with  that  of  the  former.  And  to  this  difference  is 
due  the  wide  difference  of  effects  caused  by  electricity  produced  in 
these  two  ways. 

For  example,  it  has  been  demonstrated  that  a  miniature  voltaic  bat- 
tery, consisting  of  two  wires,  the  one  of  zinc,  the  other  of  platinum, 
five-eighths  of  an  inch  long  and  one-eighth  of  an  inch  diameter,  excited 
by  one  drop  of  sulphuric  acid  mixed  with  four  ounces  of  water,  will 
liberate  as  much  electricity  in  three  seconds  of  time,  as  can  be  obtained 
by  an  electric  battery,  having  3,500  square  inches  coated  surface,  and 
charged  by  30  revolutions  of  a  plate-glass  machine  50  inches  in  diam- 
eter. This  quantity  of  electricity,  in  a  state  of  tension  given  by  the 
machine,  would  kill  a  small  animal,  and  yet  it  is  evolved  by  the  solu- 
tion of  almost  an  inappreciable  portion  of  the  zinc- wire. 

APPLICATIONS  OF  VOLTAIC  OR  GALVANIC  ELECTRICITY. 

Effects  of  the  Voltaic  Battery. 

745.  The  effects  of  the  voltaic  battery  may  be  divided  into 
Physical,  Chemical,  Physiological,  and  Magnetic. 

The  effects  of  dynamical  or  voltaic  electricity  are  all  due  to  the 
re-combination  of  the  two  fluids,  as  in  statical  or  frictional  electricity ; 
but  they  are  more  energetic,  because  of  their  continuous  action. 


390  ELECTRICITY. 

Physical  Effects. 

746.  Figure  39.— Illuminating  effects.— If  the  electrodes  of 
a  powerful  battery  be  terminated  with  points  of  well-burned  charcoal, 
and  brought  insensibly  near  to  each  other,  the  points  will  immediately 
become  incandescent,  emitting  a  light  of  dazzling  brightness.     If  the 
points  are  slightly  separated,  the  current  still  continues  to  pass  between 
them,  and  the  light  takes  the  form  of  a  luminous  arch,  called  the 

voltaic  arch  (747). 

The  figure,  taken  together 
with  Fig.  38,  will    serve    to 
illustrate   the   apparatus   em- 
ployed for  illuminating  streets, 
parks,     etc.,     with     electrical 
light.     A  suitable  column,  T, 
supports  the  electrodes  of  the 
battery  ;  the  wires  being  insu- 
lated with  gutta-percha  cover- 
ings.    As  one  of  the  charcoal 
points     slowly    wastes    away, 
while  the   other  is  somewhat 
elongated,  provision  is  made, 
by  means  of  clock-work  (not 
shown),  for  keeping  the  points 
properly  adjusted. 
An   electrical  light,  furnished  by  a  battery  of  48  carbon  couples, 
equals  that  of  572  wax  candles ;  the  light  produced  by  100  couples 
dazzles  the  eyes ;  and  that  furnished  by  600  couples  is  so  intense,  that 
it  is  as  impossible  to  look  at  it  as  it  is  to  look  at  the  noonday  sun. 

747.  Figure  40.— The  voltaic  arch.— The  figure  represents 
the  charcoal  points  which  are  connected  with  the  battery  (746).     The 
curved  lines  show  the  form  of  the  arch  of  electrical  flame,  a  white  and 

FIG.  40. 


violet  light  of  intolerable  brightness,  several  inches  in  length,  if  the 
battery  is  very  powerful. 

As  this  flame  is  even  more  brilliant  in  a  vacuum  or  in  nitrogen  or  in 
carbonic  acid,  it  follows  that  it  cannot  be  produced  by  the  combustion 
of  the  carbon  electrodes.  The  action  is  accompanied  by  a  hissing  or 


ELECTRICITY.  391 

rushing  sound,  caused,  doubtless,  by  the  removal  and  transportation  of 
particles  of  carbon  from  the  positive  to  the  negative  electrode. 

748.  Figure  41.— The  oval  form  of  the  arch.— If  the  car- 
bon electrodes  are  vertical,  and  the  negative  one  uppermost,  the  arch 
will  take  an  oval  form,  as  shown.     By  inspecting 

this  oval,  through  colored  glasses,  the  particles 
of  carbon  can  be  seen  moving  from  the  positive 
to  the  negative  electrode.  When  the  image  is 
projected  on  a  screen,  the  growth  of  the  negative 
and  the  decrease  of  the  positive  electrode  can  be 
easily  observed.  The  negative  carbon  seems  to 
glow  first,  .but  soon  the  positive  becomes  and  re- 
mains the  brightest  part  of  the  light.  There  is 
also  seen  in  this  form  of  the  arch,  a  certain  struc- 
ture, in  zones  or  bands  of  different  brilliancy,  as 
shown  by  the  plain  lines  in  the  figure. 

The  arch  is  magnetic,  that  is,  capable  of  influencing  the  magnet.  If 
a  magnet  be  brought  near  to  the  oval  arch,  the  flame  is  deflected  to  one 
side,  as  shown  in  Fig.  43.  • 

749.  Figure  42.— The  shape  of  the  electrodes.— If  the 

carbon  electrodes  be  shaped,  at  first,  both  alike,  as  shown  in  Pig.  40, 
they  will  gradually  take  different  shapes ;  the  positive  one 
taking  the  form  of  a  cup,  and  the  negative  one  remaining 
pointed,  as  shown  in  the  figure. 

750.  Properties  of  the  electrical  light.— The 

intensity  of  the  electrical  light  depends  more  on  the  size  of 
the  individual  couples  or  members  of  the  pile  than  on  their 
number.  The  light  is  unpolarized ;  it  explodes  a  mixture 
of  hydrogen  and  chlorine;  acts  on  chloride  of  silver  and 
other  photographic  preparations  like  the 
sun.  Daguerreotypes  are  taken  with  it; 
and  for  taking  microscopic  photographs, 
it  is  preferable  to  solar  light. 

751.  Figure  43.— Heating  effects.— Defla- 
gration.— The  heat  produced  by  a  powerful  bat- 
tery, say  of  600  couples,  is  so  intense  that  even  pure 
carbon  has  been  softened  by  its  power.  When  a  cur- 
rent of  voltaic  electricity  passes  through  a  conductor, 
it  heats  it ;  and,  according  to  the  power  of  the  bat- 


392 


ELECTRICITY. 


tery,  it  becomes  fused  or  even  vaporized.      Small  wires   burn  with 
splendid  brilliancy. 

When  the  positive  electrode  is  formed  into  a  small  crucible  of  carbon, 
S,  as  shown  in  the  figure,  gold,  silver,  platinum,  and  other  substances, 
are  readily  fused,  deflagrated,  or  volatilized.  Silver  burns  with  a  green- 
ish, and  gold  with  a  bluish-white,  light.  Platinum,  infusible  in  the 
hottest  furnace,  melts  into  spherical  globules  with  a  dazzling  light. 

Chemical  Effects. 

752.  Decomposition. — The  most  important  chemical  effects  of 
voltaic  or  galvanic  electricity,  are  the  decomposition  of  bodies  traversed 
by  it,  and  the  transportation  of  their  elements.     Decomposition  by 
means  of  electricity  is  one  of  the  most  valuable  modern  discoveries, 
yielding  some  of  the  richest  gifts  which  abstract  science  ever  bestowed 
upon  the  practical  arts  of  life. 

753.  Figure  44.— Method  of  electrotyping. — This  is  a  pro- 
cess of  casting  metals  without  heat ;  or  the  copying  of  medals,  statues, 
type,  and  the  like,  in  metal,  by  the  aid  of  voltaic  electricity. 

FIG.  44. 


The  battery  shown  in  the  figure  differs  from  the  carbon  battery, 
Fig.  37  (739),  by  having  a  cylinder  of  zinc  substituted  for  the  carbon, 
and  a  cylinder  of  copper  in  place  of  the  zinc.  The  outer  vessel  is  of 
glass,  and  is  filled  with  a  solution  of  copper  (blue  vitriol),  whic^  is 


ELECTRICITY.  393 

kept  saturated  by  crystals  of  the  sulphate,  E,  placed  in  the  bottom  of 
the  vessel.  The  porous  vessel  (containing  the  zinc)  is  filled  with  dilute 
sulphuric  acid  (oil  of  vitriol). 

The  oxygen,  resulting  from  the  decomposition  of  the  water,  goes  to 
the  zinc,  forming  oxyde  of  zinc,  which  is  dissolved  by  the  sulphuric 
acid,  giving  sulphate  of  zinc.  The  hydrogen  goes  to  the  sulphate  of 
copper  and  decomposes  it.  These  chemical  actions  keep  up  the  electric 
current,  as  shown  by  the  arrows,  which  will  continue  as  long  as  the 
outer  vessel  is  supplied  with  saturated  solution  of  sulphate  of  zinc. 

Preparing  the  mould.— To  produce  a  metallic  duplicate  of  an 
object,  as  a  medal,  a  wood-cut,  or  a  form  of  type,  it  is  first  necessary  to 
prepare  an  accurate  mould  of  the  object,  made  of  some  material,  as 
plaster,  wax,  or  gutta-percha,  which  will  resist  the  action  of  the  acids 
employed.  Powdered  black-lead  is  first  rubbed  on  the  wood-cut  or 
other  object,  to  prevent  the  warm  wax,  or  other  plastic  substance,  from 
sticking.  The  wax  is  then  pressed  upon  the  engraving,  or  object  to  be 
copied,  until  it  touches  every  part.  After  hardening,  the  mould  is 
removed,  and,  to  render  it  a  good  conductor  of  electricity,  it  is  coated 
with  powdered  black-lead.  The  mould,  thus  prepared  to  receive  the 
metal,  is  made  ready  for  the  bath  by  attaching  it  to  suspending  wires, 
as  shown  in  the  upper  part  of  the  figure. 

Method  of  depositing  the  metal  upon  the  mould.— A  is  a 

vessel  filled  with  a  solution  of  sulphate  of  copper ;  T  and  H  are  metallic 
rods  communicating  with  the  two  poles  of  the  battery;  the  mould  is 
suspended  from  the  rod,  H,  and  facing  it  is  a  plate  of  pure  copper,  sus- 
pended from  the  rod,  T.  The  mould  and  the  plate  of  copper  constitute 
the  two  electrodes,  the  mould  being  the  negative  one. 

The  electric  current  of  the  battery  passing  through  the  solution, 
between  the  copper  plate  and  mould,  decomposes  the  sulphate  into 
sulphuric  acid,  oxygen,  and  pure  copper.  The  acid  and  oxygen  go  to 
the  positive  electrode,  and,  uniting  with  the  copper  plate,  produce  sul- 
phate of  copper ;  the  copper  goes  to  the  negative  electrode  and  is  there 
deposited  011  the  mould.  In  two  days,  or  so,  the  coating  of  copper  is 
sufficiently  thick  to  be  removed  from  the  the  wax  mould.  If  the  mould 
be  perfect  the  casting  will  be  a  perfect  fac-simile  of  the  object. 

If  the  object  copied  be  a  form  of  type,  or  a  wood-cut,  the  metallic 
copy,  being  nailed  to  a  wooden  block,  will  serve  to  print  from  100,000 
to  200,000  impressions. 

The  positive  electrode  should  be  of  the  same  metal  as  that  in  solu- 
tion, and  as  large  as  the  surface  to  be  coated,  and  these  should  not  be 
larger  than  the  plates  of  the  battery  furnishing  the  current. 


394 


ELECTRICITY. 


754.  Electro-gilding  and  electro-plating  consist  in  cover- 
ing bodies,  as  spoons,  watch-cases,  etc.,  with  gold,  silver,  etc.,  by  a  pro- 
cess similar  to  that  of  electrotyping.     The  object  to  be  plated  is  first 
thoroughly  cleansed  and  then  suspended  in  a  solution  of  the  metal 
with  which  the  object  is  to  be  plated. 

755.  Figure   45.— Voltaic    decomposition    of  water.— 
Water  is  composed  of  oxygen  and  hydrogen  gases,  in  the  proportions 

FIG  45<  of  one  measure  of  the  former  to  two  of  the 

latter. 

The  process  of  decomposing  water  by  a 
voltaic  current  is  quite  simple.  Two  glass 
tubes,  filled  with  water,  are  inverted  in  a 
vessel  of  water.  The  vessel  has  a  wooden 
bottom,  through  which  the  electrodes  of  the 
battery  are  passed,  so  as  to  enter  the 
mouths  of  the  tubes,  as  seen  in  the  figure. 
To  increase  the  conducting  power  of  the 
water,  a  small  quantity  of  sulphuric  acid 
is  added.  The  electrodes  are  platinum  or 
gold,  otherwise  the  oxygen  would  combine 
with  one  of  them.  When  the  current 
passes  from  one  electrode  to  the  other, 
through  the  water,  the  decomposition  be- 
gins, as  shown  by  the  bubbles  of  gas  rising 

in  the  two  tubes.     The  oxygen  rises  in  the  tube  0  over  the  positive 

electrode,  and  the  hydrogen  in  the  tube  H  over  the  negative  electrode. 
The  gases  are  pure,  and  their  volumes  are  as  1  to  2,  as  indicated  by 

the  height  of  the  water  in  the  tubes. 

The  rapidity  of  the  decomposition  depends  upon  the  intensity  of  the 

current. 

756.  Figure  46.— Decomposition  of  salts.— Fill  a  glass 
tube,  as  represented,  with  a  solution  of  some  neutral  salt  (as  sulphate 
of  soda),  and  color  the  solution  with  an  infusion  of  litmus  (blue  cab- 
bage), then  pass  a  voltaic  current  through  the  saline  solution,  by  dipping 
the  platinum  electrodes  into  the  two  arms.     The  dissolved  salt  will  be 
decomposed,  the  acid  constituent  passing  to  the  positive  electrode  in 
the  arm  R,  and  the  alkaline  to  the  negative  electrode  in  the  arm  K, 
as  will  be  shown  by  the  infusion,  which  turns  re<J  by  the  acid,  and 
green  by  the  alkali. 

The  whole  liquid  will  be  turned  to  red  and  green.     The  arrows  indi- 
cate the  passage  of  the  constituents.     If  the  positive  electrode  be  placed 


ELECTRICITY. 


395 


in  the  arm  K,  and  the  negative  in  the  FIG.  46. 

arm  R,  the  constituents  of  the  salt  will 

be  transposed  in  the  tube,  as  shown  by 

the  red  turning  to  green  in  the  arm  R, 

and  the  green  turning  to  red  in  the  arm 

K. 

In  all  decomposition  of  substances 
containing  acid  and  an  alkali,  the  acid 
appears  at  the  positive  and  the  alkali  at 
the  negative  electrode. 

In  all  reduction  of  the  metals  from  the 
solution  of  a  metallic  salt,  the  acid  appears 
at  the  positive,  and  the  metal  at  the  nega- 
tive electrode.  That  is,  oxygen  and  the 
acids  appear  at  the  positive,  hydrogen  and 
the  metals  at  the  negative  electrode. 

Whenever  a  compound  is  decomposed  by  electricity,  electro-negative 
elements  appear  at  the  positive,  and  the  elector-positive  at  the  negative 
electrode. 


757.  The  quantity  of  electricity  required  to  produce 
chemical  action  is  enormous. — It  has  been  demonstrated  that  it 
requires,  to  decompose  one  grain  of  water,  an  amount  of  frictional 
electricity  equal  to  that  furnished  by  the  discharge  of  an  electric  pane 
having  thirty- two  acres  of  surface — "  equal  to  a  very  powerful  flash  of 
lightning." 


Physiological  Effects. 

758.  The  physiological  effects  of  the  voltaic  current 

depend  upon  the  number  of  the  elements  or  pairs,  rather  than  their 
size.  No  sensation  is  felt,  with  dry  hands,  from  one  or  even  a  small 
number  of  pairs.  From  fifteen  carbon  couples  a  smart  twinge  is  felt, 
reaching  to  the  shoulders.  The  sensation  is  not  like  that  produced  by 
statical  electricity,  being  continuous  and  less  severe.  It  is  only  at  the 
making  and  breaking  of  the  contact  that  the  shock  is  felt.  The  cur- 
rent from  a  very  powerful  battery  becomes  painful  and  dangerous,  and 
even  fatal. 

The  effect  of  the  voltaic  current  on  bodies  of  dead  animals  is  pecu- 
liarly striking.  It  causes  violent  contractions  of  the  muscles,  similar 
to  those  of  living  beings.  Experiments  have  been  performed  upon  the 
dead  bodies  of  criminals,  which  resulted  in  causing  the  lungs  to  act, 
the  eyes  to  open,  the  lips  to  move,  the  body  to  writhe,  and  the  face  to 


396  ELECTRICITY. 

contort,  assuming  expressions  strange  and  horrifying  beyond  descrip- 
tion. 

Contact  with  but  one  of  the  poles  of  the  battery  produces  no  effect. 

A  gentle  current  hastens  the  germination  of  seeds  and  growth  of 
plants. 


OHAPTEE   XYI. 

ELECTRO-DYNAMICS. 
ELECTRO-MAGNET  IS  It. 

759.  Relation  between  magnetism   and  electricity.— 

Magnetic  and  electric  fluids  have  many  analogous  properties.  In  each 
case  fluids  of  the  same  name  repel,  whilst  those  of  an  opposite  name 
attract.  A  stroke  of  lightning  often  reverses  the  poles  of  magnets,  and 
sometimes  destroys  magnetism.  They  have  also  points  of  dissimilarity. 
Magnetic  fluids  are  not  transmitted  through  conductors  as  electrical 
fluids  are.  Magnets  do  not  return  to  a  neutral  state  when  brought 
into  contact  with  the  earth.  Magnetism  can  only  be  developed  in  a 
few,  whereas  electricity  may  be  developed  in  all  bodies. 

With  such  analogies  on  the  one  hand,  and  dissimilarities  on  the 
other,  nothing  conclusive  could  be  affirmed  respecting  the  identity  of 
these  two  wonderful  agents,  until,  in  1819,  Ersted  discovered  that  they 
are  intimately  allied,  if  not  identical. 

760.  Ersted' s   discovery. — Though   many  philosophers  had 
sought  to  evolve  the  phenomena  of  magnetism  from  the  voltaic  battery, 
they  had  experimented  without  connecting  the  poles.     Ersted  simply 
closed  the  battery  circuit  by  a  conductor ;  and  discovered  at  once  the 
important  fact  that  wire  (of  whatever  metal),  connecting  the  poles,  acts 
upon  the  magnetic  needle  as  if  the  wire  itself  were  a  magnet. 

This  constitutes  the  discovery  of  the  fundamental  principle  of  electro- 
magnetism. 

761.  Figure  47.— Action  of  an  electric  current  upon  a 
magnet  or  needle. — If  positive  electricity  flows  from  south  to  north 
over  a  horizontal  wire  placed  in  the  magnetic  meridian,  a  needle  would 
have  its  north  end  deflected  to  the  west,  if  it  is  placed  below  the  wire ; 
and  to  the  east,  if  placed  above  the  wire.     If  the  needle  is  placed  on  the 
east  side  of  the  wire,  its  north  end  is  depressed,  if  on  the  west  side  of 


ELECTRICITY. 


397 


the  wire,  the  north  end  of  the  needle  is  raised.     If  the  current  passes 
from  the  north  to  the  south,  the  movements  of  the  needle  are  all  reversed. 

By  means  of  the  rectangle  sur-  JTIG> 

rounding  the  needle,  in  the  figure, 
the  current  can  be  sent  above,  or 
below,  or  above  and  below,  the 
needle,  by  changing  the  conjunc- 
tive wires  in  the  connecting  sock- 
ets at  N  and  S. 

If  the  current  passes  around  above  and  below  the  needle,  in  opposite 
directions,  the  opposite  currents,  instead  of  neutralizing,  will  assist 
each  other,  and  the  needle  will  move  in  accordance  with  the  first  di- 
rection of  the  current.  Galvanometers  are  constructed  upon  this 
principle. 

762.  Figure  48.— Galvanometers   or  multipliers.— Gal- 
vanometers are  'instruments  for  measuring  the  force  of  electric  currents. 
As  just  stated,  the  force  exerted  FlG 

upon  the  needle  is  greater  when 
the  current  is  passed  once  around, 
instead  of  simply  over  or  under 
it.  It  is  also  true  that  the  force 
is  multiplied  in  proportion  to 
the  number  of  times  the  con- 
ducting wire  is  passed  around 
the  needle. 

The  figure  represents  a  coil  of  copper  wire,  making  thirty  or  forty 
convolutions  around  the  needle,  N,  the  wire  being  first  covered  with 
silk  or  cotton,  like  common  bonnet-wire,  for  the  purpose  of  insulating 
it.  A  graduated  circle  is  fixed  on  the  stand  or  bottom  board,  to 
measure  the  amount  of  the  deflection.  The  coupling  sockets  serve  to 
connect  the  ends  of  the  coiled  wire  with  the  poles  of  the  battery,  the 
arrows  indicating  the  direction  of  the  current. 

By  this  instrument  a  feeble  current  becomes  quite  sensible.  For 
particular  purposes,  and  when  the  current  is  very  feeble,  many  thou- 
sand convolutions  of  fine  wire  are  employed. 

763 .  The  directive  action  of  the  earth. — In  all  experiments 
the  needle  is  more  or  less  governed  by  the  magnetic  force  of  the  earth, 
which  must  be  neutralized  in  the  experimental  needle  in  order  to  ac- 
curately estimate  the  force  of  electrical  currents  ;  for,  before  the  needle 
can  be  moved  by  the  current,  it  must  first  exert  a  force  equal  to  that 
exerted  upon  it  by  the  earth's  magnetism.     This  directive  force  of  the 
earth  is  overcome  by  what  is  called  the  astatic  needle. 


398  ELECTRICITY. 

764-  Figure  49.— The  astatic  needle  consists  of  two  needles, 
placed  and  held  one  above  the  other,  with  their  poles  reversed,  as  indi- 
go 49  cated  by  the  signs  plus  and  minus. 
This  needle,  as  shown  in  Fig.  56 
(634),  is  suspended  by  a  fine  fibre 
of  raw  silk.  The  two  needles  are 
made  so  as  not  to  quite  neutralize 
each  other,  thus  giving  the  system 
a  slight  directive  force.  While  this 
needle  is  thus  rendered  nearly  neutral  as  to  the  earth's  magnetism,  it 
is  not  thereby  rendered  less  responsive  to  the  electric  current,  as  both 
needles  tend  to  turn  in  the  same  direction,  in  consequence  of  one  being 
within  and  the  other  above  the  coil  or  bend  of  the  wire.  Hence,  the 
astatic  needle  is  indifferent  to  the  influence  of  the  earth,  and  very  sen- 
sitive to  electric  currents. 

765.  The  electro-magnetic  force  is  exerted  in  a  lateral 
and  tangential  direction  to  the  electric  current.    The  electro- 
magnetic current  or  force  moves  at  right  angles  to  the  course  of  the 
conjunctive  wire. 

A  sewing  needle  or  a  bar  of  soft  iron,  held  vertically  on  one  side  of 
the  wire,  instantly  becomes  a  magnet,  with  its  north  pole  toward  the 
earth.  If  the  bar  or  needle  be  held  vertically  on  the  other  side  of  the 
wire,  its  polarity  is  instantly  reversed.  If  the  bar  be  revolved  around 
the  wire  in  a  vertical  plane,  at  right  angles  to  the  wire  or  current,  it 
retains  its  polarity  in  every  position.  If  it  be  a  steel  bar,  it  retains  its 
magnetism  after  the  current  ceases. 

The  relation  of  the  electro-magnetic  and  the  electric  currents,  above 
explained,  will  be  more  easily  remembered  if  thus  stated :  Suppose  the 
positive  electric  current  or  ivire  to  enter  the  feet  and  pass  out  of  the 
head  of  the  observer,  his  face  being  turned  toward  the  magnet,  then  the 
north  pole  of  the  magnet  is  invariably  deflected  to  the  left. 

When  a  magnetic  pole  is  influenced  by  an  electric  current,  it  does 
not  move  either  directly  toward  or  directly  from  the  conducting  wire, 
but  it  tends  to  rotate  around  it,  showing  that  the  magnetic  force  of  the 
electric  current  is  exerted  in  a  direction  tangential  to  the  conducting 
wire  or  current. 

766.  Ampere's  electro-magnetic  theory  supposes  magnet- 
ism to  be  due  to  currents  of  electricity  flowing  around  the  ultimate 
molecules  of  a  magnet,  always  in  the  same  direction.     Or,  differently 
stated,  we  may  imagine  each  of  the  magnetic  molecules  to  be  replaced 
by  a  conjunctive  wire  bent  on  itself,  in  which  a  constant  current  of 


ELECTRICITY. 


399 


electricity  is  maintained,  as  from  a  battery.  The  interior  currents  neu- 
tralizing each  other,  the  total  effect  is  the  same  as  that  of  a  set  of  sur- 
face currents  flowing  around  the  magnet,  in  the  direction  of  the  hands 
of  a  watch,  if  the  south  end  of  the  magnet  is  placed  against  the  back  of 
the  watch,  all  acting  at  right  angles  to  the  axis  of  the  magnet. 

Hence,  as  the  magnetic  needle  strives  to  place  itself  at  right  angles 
to  the  path  of  the  current  on  the  conjunctive  wire,  it  follows  that  cur- 
rents of  the  magnet  seek  a  parallelism  to  that  in  the  conjunctive  wire. 

767.  Figure  50. — Mutual  action  of  electric  currents. — 

In  accordance  with  the  theory  just  stated,  parallel  currents  attract  each 
other  when  they  flow  in  the  same  direc-  p 

tion ;  and  repel  each  other  when  they 
flow  in  opposite  directions.  These 
facts  may  be  demonstrated  by  means 
of  a  floating  current,  which  may  be 
produced  by  fixing  a  piece  of  zinc  and 
a  piece  of  copper,  Z  and  C,  in  a  disk 
or  float  of  cork,  and  connecting  the 
metals  with  a  wire ;  placing  the  whole  in  a  dish  of  acidulated  water,  as 
shown.  Along  the  conjunctive  wire  will  flow  an  electric  current  from 
the  copper  to  the  zinc.  If  now  a  conjunctive  wire  of  a  battery  be 
stretched  between  the  two  hands,  and  held  parallel  to  the  conjunctive 
wire  of  the  floating  couple,  as  shown  by  the  long  arrow,  with  the  cur- 
rents flowing  in  opposite  directions,  as  indicated  by  the  several  arrows, 
the  repulsion  will  be  manifested  by  the  movement  of  the  floating 
couple.  If  either  current  be  reversed,  so  that  they  will  flow  in  the  same 
direction,  the  conjunctive  wire  of  the  floating 
couple  will  assume  a  position  parallel  to  the  con- 
junctive wire  of  the  battery. 


FIG.  51, 


768.  Figure  51.— Attraction  of  cur- 
rents shown  by  the  oscillating  spiral.— 

The  attraction  of  currents,  flowing  in  the  same 

direction,  is  neatly  illustrated  by  the  spiral  wire, 

suspended  as  shown,  and  dipping  into  a  glass 

of  mercury.     Below  the  platform  the  two  poles 

of  a  battery  are  brought,  one  in  contact  with 

the  mercury  and  the  other  with  the  metallic 

standard.     As  the  current  flows  over  the  wire, 

as  indicated  by  the  arrows,  each  turn  of  the 

spiral  attracts  the  next  turn,  shortening  the  spiral,  and  breaking  the 

mercurial  connection,  which  causes  a  spark.     The  attraction  ceasing 


400 


ELECTRICITY. 


FIG.  52. 


by  the  current  being  broken,  the  weight  of  the  spiral  again  restores 
the  connection,  and  so  on,  causing  a  continuous  movement  and  emis- 
sion of  the  spark. 

769.  Figure  52.— Action  of  magnets  upon  currents.— It 

has  been  shown  that  electric  currents  exercise  a  directive  force  not  only 

upon  magnets,  but  also  upon  each  other. 
This  figure  will  illustrate  that  magnets  exert 
a  directive  force  upon  currents. 

A  copper  wire,  S,  is  bent  into  a  circular 
form,  and  suspended  to  the  two  arms  of  the 
supports,  as  shown.  The  ends  of  the  wire 
are  tipped  with  steel,  and  rest  in  the  cups 
of  mercury,  H  and  L,  so  that  the  wire  hoop 
is  free  to  revolve  around  the  vertical  line 
passing  through  the  points.  The  wires  of  a 
battery,  passing  through  the  sockets,  con- 
nect, under  the  platform,  with  the  sup- 
ports and  mercurial  cups,  H  and  L.  On 
completing  the  circuit,  the  current  flowing 
as  indicated  by  the  arrows,  the  plane  of  the  hoop,  S,  will  assume  the 
east  and  west  direction,  and  come  to  rest  at  right  angles  to  the  mag- 
netic meridian. 

If  a  bar-magnet  be  held  horizontally  within  the  hoop,  the  axis  of  the 
magnet  being  in  the  plane  of  the  circle,  the  hoop  will  turn  around  and 
come  to  rest  at  right  angles  to  the  axis  of  the  magnet. 

This  experiment,  made  first  by  Ampere,  is  the  reverse  of  Ersted's 
experiment  and  discovery  (760). 

770.  Figure  53.— A  single  helix. — If  the  conjunctive  wire  be 
wound  into  a  helix,  as  shown,  and  a  current  passed  over  it  in  the  direc- 
tion indicated  by  the  arrow-points,  the  effects  of  the  current,  from 
K  to  L,  will  be  neutralized  by  its  return  from  L  to  Y,  and  there  will 

FIG.  53. 


remain  only  the  effect  due  to  its  spiral  revolution  about  LY.    The 
effect  of  the  helix  thus  wound,  is  reduced  solely  to  the  influence  of  a 


series  of  equal  and  parallel  circular  currents, 
is  called  a  solenoid. 


This  form  of  the  wire 


ELECTRICITY  401 

771.  Figure   54.— A  double  helix.— If   a  silk-covered  wire 
be  coiled  into  a  double  helix,  as  represented  in  this  figure,  and  its  ends 
tipped  with  steel  points,  P,  and  set  into  the  cups  of  mercury  (Fig.  52), 
the  coil  will  be  free  to  rotate  in  a  FlG  54 
horizontal    plane.     Thus    placed, 

and  the  current  passed  over  it,  in 
the  direction  indicated  by  the  ar- 
rows, it  will  assume  the  north 
and  south  direction,  with  the  end 
0  to  the  north. 

It  takes  this  direction   because 
in  no  other  position  would  the  currents  pass  at  right  angles  to  the 
magnetic  meridian  of  the  earth  (769). 

The  solenoid,  therefore,  simulates,  in  all  respects,  the  character  of  a 
magnetic  needle,  although  possessing  not  a  particle  of  iron  or  steel  in 
its  composition. 

If  another  solenoid,  having  a  current  passing  over  it,  be  presented  to 
this  one,  they  will  manifest  all  the  phenomena  of  attraction  and  repulsion, 
in  the  same  manner  as  if  the  two  helices  or  solenoids  were  magnets. 

772.  Figure  55.— Magnetizing  by  the  helix  and  elec- 
trical current. — If  a  bar  of  iron  or  steel  be  placed  within  a  helix 
of  wire,  as  shown,  and  an  electric  current  passed  over  the  wire,  the  bar 
instantly  becomes  a  magnet.     If  the  bar  is  soft 

iron  it  loses  its  magnetism  the  instant  the  cur- 
rent is  broken ;  if  it  be  steel,  it  retains  its  mag- 
netism. 

Insulated  wire  is  employed  for  this  purpose, 
and  the  coils  repeated  one  over  another,  in  order 
to  multiply  the  convolutions. 

Determining  the  poles. — If  the  bar  stands  on 
the  floor  or  table  and  the  current  flows  from 
plus  (  +  )to  minus  (  — ),  beginning  at  the  top  and 
moving  around  the  bar  in  the  direction  that  the 
hands  of  a  watch  move,  the  N  pole  will  be  up- 
permost, as  shown  in  the  figure.  If  the  wire  is 
wound  in  the  opposite  direction,  the  S  pole  will 
be  uppermost.  Magnets  may  be  thus  made  with  statical  electricity. 

The  explanation  of  this  is,  that  each  volute  of  the  helix,  carrying 
an  electric  current,  is  itself  an  active  magnet ;  hence,  under  the  united 
influence  of  a  great  number  of  such  circular  and  parallel  currents  or 
magnets,  the  coercitive  force  (627)  of  the  bar  is  decomposed  and  active 
magnetism  is  induced. 

26 


402  ELECTRICITY. 


Figure  56.  —  Electro  -magnets  are  masses  of  soft  iron 
wound  with  coils  of  insulated  copper  wire.     They  vary  in  form  and 
FIG.  56.  size.     The  figure  represents  the  form  of 

those  designed  to  sustain  great  weights. 
The  spools,  J  and  H,  are  virtually  one  ; 
the  direction  of  the  whorl  is  only  appa- 
rently reversed. 

.The  surprising  power  of  the  horse- 
shoe electro-magnet  is  developed  only 
when  the  armament  is  in  contact  with 
the  poles.  Their  polarity  is  reversed, 
instantly,  by  reversing  the  poles  of  the 
battery. 

Magnets  of  this  kind  are  made  that 
support  3,000  pounds  or  more  ;  yet  this 
enormous  power  can  be  alternately  induced  and  paralyzed  any  number 
of  times,  and  with  inconceivable  rapidity,  by  merely  moving  the  end 
of  a  small  wire  through  an  almost  inappreciable  space. 

774-  Bodies  suspended  without  contact.  —  If  one  end  of 

a  bar  of  iron  be  brought  near  to  one  extremity  of  a  longitudinal  helix, 
vertically  placed,  and  connected  with  the  battery,  it  will  be  attracted  by 
and  drawn  into  the  helix,  where  it  will  remain  suspended  without  visi- 
ble contact  or  visible  support,  so  long  as  the  current  is  passing.  Iron 
bars  weighing  nearly  100  pounds  have  been  thus  suspended  in  the  air. 
Of  course  they  fall  the  instant  the  current  is  broken. 

775.  Utilization  of  electro  -magnetic    force.  —  Many    at- 
tempts have  been  made  to  utilize  this  very  controllable  force,  by  apply- 
ing it  as  a  motive-power  ;  but  the  force  (diminishing  even  more  rapidly 
than  the  square  of  the  distance  from  the  magnet  increases)  acts  through 
such  a  limited  distance,  that  no  important  results  have  been  achieved 
in  its   application,  where  much  power  is  required.     Jacobi   expended 
$120,000,  granted  him  by  the  Russian  government,  in  experimenting 
for  this  purpose.     The  most  valuable  utilization  of  the  electro-magnet 
is  made  in  its  application  to  the  electric  telegraph. 

The  Electric  Telegraph. 

776.  First  experiments  in  electrical  telegraphing.—  The 

observation  of  a  simple  phenomenon  or  the  discovery  of  a  single  fact 
should  not  entitle  the  observer  or  discoverer  to  all  the  credit  which 
mankind  are  ever  ready  to  bestow  for  the  achievement  of  great  results; 


ELECTRICITY.  403 

for  the  discovery  and  practical  application  of  all  the  general  principles 
necessary  to  the  accomplishment  of  any  great  good,  are  seldom,  if  ever, 
the  result  of  single-handed  efforts. 

For  example,  in  1747,  Watson  employed  frictional  electricity,  and 
transmitted  messages  over  a  single  wire  two  miles  or  more  in  length, 
the  wires  being  attached  to  chimney-tops.  We  now  speak  of  Morse's 
electric  telegraph  worked  by  galvanic  batteries ;  yet,  Galvani  never  saw 
a  galvanic  battery,  nor  had  he  any  knowledge  of  such  a  thing.  All 
along  from  Watson's  time  to  the  date  of  Morse's  improvements,  exten- 
sive efforts  were  made,  in  several  countries,  to  utilize  the  electric  cur- 
rent by  its  application  to  telegraphy.  Since  Morse's  improvements 
were  made,  many  other  useful  and  minor  improvements  have  been  in- 
vented. 

The  distinctive  feature  of  Morse's  invention,  first  employed  in  1844, 
consists  in  permanently  recording  the  message  on  paper,  instead  of  in- 
dicating it  by  such  signals  as  require  to  be  observed  at  the  instant  they 
are  made  or  not  at  all. 

An  electrical  signal  telegraph  was  employed  in  France,  for  ordinary 
purposes,  until  it  was  replaced  by  Morse's  registering  apparatus. 

777.    Figure    57.— Morse's    recording    telegraph. — The 

electro-telegraphic  device  embraces — 

First,  an  apparatus  for  producing  a  force,  by  which  mechanical  results 
can  be  produced  and  controlled  at  a  great  distance.  This  apparatus 
consists  of  the  battery,  the  long  wires,  and  the  electro-magnet. 


Second,  two  other  instruments,  one  for  dispatching  the  message,  and 
the  other  for  receiving  or  recording  the  message. 

The  receiving  or  recording  instrument.     This  consists  essentially  of 


404 


ELECTRICITY. 


a  simple  lever,  L,  called  the  pen-lever,  provided  with  a  soft  armature, 
B,  arranged  over  the  poles  of  an  electro-magnet,  N.  This  lever,  L,  can 
be  worked  on  its  centre  or  fulcrum  by  an  operator  situated  in  an  office 
hundreds  of  miles  away,  by  his  completing  and  breaking  the  circuit. 
In  the  opposite  end  of  the  pen-lever,  is  fixed  a  point,  F,  called  the  pen. 
HH  are  a  pair  of  rollers,  which  slowly  revolve  with  uniform  velocity, 
by  means  of  clock-work.  The  rollers  draw  between  them  a  fillet  of 
paper,  from  the  roll  R.  As  the  paper  passes  along,  the  pen,  F,  at  the 
will  of  the  distant  operator,  is  made  to  puncture  it,  leaving  the  pin- 
hole  mark.  If  the  operator  allows  the  current  to  flow  for  an  instant, 
the  pen  will  make  an  elongated  impression  or  mark  on  the  paper.  So, 
by  varying  the  length  of  time  during  which  the  current  is  interrupted, 
various  marks,  which  represent  the  letters  of  the  alphabet,  are  made. 
Some  of  these  marks  are  seen  on  the  fillet  of  paper  at  D.  A  feeble 
spring,  S,  serves  to  draw  the  pen  from  the  paper,  and  lift  the  armature, 
B,  from  the  magnet.  The  current  is  received  by  the  wire  A,  and  re- 
turned by  the  wire  G,  which  passes  into  the  earth,  the  earth  serving  as 
the  return  wire. 

The  various  characters,  which  represent  the  letters  of  the  alphabet, 
are  shown  in  the  following  table : 

MORSE'S   ALPHABET. 


a  

\      _  L  .  __  _       

s  .  .  . 

b  —  ... 

k  

t  — 

1 

d  

m  

v  .  .  •  — 

f    .  —  . 

o  .  . 

Cf 

y.  .      .... 

h  .  .  .  .  ' 

—  . 

i    .  . 

. 
r  .  .  . 

The  instrument  for  transmitting  the  message  is  a  simple 
apparatus,  consisting  of  a  light  lever  provided  with  a  suitable  knob, 
called  the  finger-fay,  for  receiving  the  pressure  of  the  fingers.  It  is  so 
arranged  that  by  gently  pressing  upon  the  knob,  the  circuit  is  com- 
pleted, and,  when  the  fingers  are  raised,  the  lever  is  lifted  by  a  spring, 
which  interrupts  the  current.  By  varying  the  length  of  time  during 
which  the  current  is  interrupted  and  closed,  all  the  above  alphabetical 
characters  can  be  made  at  the  receiving  station,  many  miles  away. 


House's  telegraph,  or  the  printing  telegraph,  differs  from 
Morse's  and  others  principally  in  an  arrangement  whereby  the  message, 


ELECTRICITY. 


405 


FIG.  58. 


as  transmitted,  is  printed  in  ordinary  capital  letters,  on  a  narrows  trip  of 
paper,  at  the  rate  of  two  or  three  hundred  letters  per  minute. 

778.  The  earth  circuit.— Watson  and  Franklin,  in  1747-8,  used 
the  earth  as  the  return  circuit,  but  they  employed  statical  or  Motional 
electricity.     Yet  it  was,  for  a  long  time,  believed  to  be  necessary,  in 
using  voltaic  electricity,  to  employ  two  wires.     Steinheil,  in  1837,  ob- 
viated the  whole  resistance  of  the  return  wire,  by  burying  a  large  plate 
of  copper  at  each  station  with  which  the  circuit  wire  communicated. 

This  method,  now  universally  adopted,  of  returning  the  current 
through  the  earth,  and  so  obviating  the  resistance  and  expense  of  the 
return  wire,  must  be  considered  one  of  the  most  important  discoveries 
in  connection  with  the  telegraph. 

Insulators. — Telegraphic  wires  are  insulated  from  the  poles,  that 
support  them  from  one  station  to  another,  by  means  of  glass  or  some 
other  non-conducting  holders. 

779.  Figure  58.— Electro-dynamic  induction.— The  revolv- 
ing electro-magnet  operates  by  virtue  of  the  attraction  of  dissimilar, 
and  repulsion  of  similar,  poles  of  magnets. 

This  instrument  consists  of  a  permanent 
U-magnet,  between  the  poles  of  which  an  elec- 
tro-magnet is  horizontally  supported  on  a  ver- 
tical spindle  passing  through  its  axis,  as  shown. 
The  wires  of  the  electric  battery  pass  through 
ivory  collars,  inserted  in  a  frame,  which  sus- 
tains the  upper  end  of  the  spindle.  By  a  sim- 
ple contrivance,  called  a  break-piece,  the  con- 
tinuity of  the  current  is  interrupted  twice  in 
every  revolution,  when  the  armature  is  in  the 
position  shown  in  the  figure.  The  magnetic 
force  being  thus  paralyzed,  the  momentum  of 
the  mass  carries  the  armature  by  the  poles  of 
the  fixed  magnet,  when  the  battery  connection 
is  again  completed. 

The  revolution  is  caused  by  the  mutual  repulsion,  and  then  the 
mutual  attraction,  between  the  two  opposite  poles  of  the  two  magnets, 
as  the  connection  is  broken  and  the  poles  of  the  electro-magnet  are 
reversed. 

The  velocity  with  which  the  electro-magnet  in  this  little  machine 
revolves,  is  from  2,000  to  3,000  revolutions  per  minute,  attended  with 
from  4,000  to  6,000  reversals  of  polarity,  and  as  many  intervals  of  ces- 
sation of  the  magnetic  force. 


406  ELECTRICITY. 

780.  Figure  59.— Cause  of  the  earth's  magnetism. — If  a 

metal  ring  be  warmed  at  one  point  only,  as  with  a  spirit-lamp,  no  elec- 
trical effect  is  produced ;  but  if  the  lamp  be  moved  along  the  ring,  an 
electric  current  is  set  up,  which  traverses  the  ring  in  the  same  direction 
the  lamp  has  taken. 

In  this  manner  the  sun  continually  heats  successive  portions  of  the 
earth,  causing  currents  of  electricity  to  flow  around  the  globe  from  east 
to  west,  in  a  direction  at  right  angles  to  the  line  joining  the  magnetic 
poles. 

A  magnetic  needle,  therefore  (as  also '  the  double  helix,  Fig.  54), 
points  north  and  south,  because  it  is  only  when  in  this  position  that  its 
electrical  currents  can  be  parallel  to  those  of  the  earth  (766-7). 

FIG.  59. 


The  figure  represents  a  small  artificial  globe,  surrounded  by  a  coil 
of  insulated  wire,  surmounted  by  a  magnetic  needle.  When  the  cur- 
rent of  the  battery  is  transmitted  through  the  wire,  the  needle  points 
to  the  north  pole  of  the  globe. 

Tlie  dip  of  the  needle  is  accounted  for  in  the  same  manner.  At  the 
polar  regions  it  dips  in  order  to  place  its  currents  parallel  with  those 
of  the  earth. 

781.  Figure  60. — Magneto-electricity.  —  Electricity  gene- 
rated by  a  magnetized  bar  is  called  magneto-electricity.  If  one  of  the 
poles  of  a  powerful  bar-magnet,  A,  be  introduced  within  a  helix  of  in- 
sulated wire,  S,  an  electric  current  will  be  excited  in  the  wire  every 
time  the  magnet  enters  or  leaves  the  coil.  The  current  excited  by  one 
pole  of  the  magnet  will  flow  in  the  opposite  direction  to  that  induced 
by  the  opposite  pole.  An  electro-magnet  will  produce  the  same  results, 


ELECTRICITY. 


407 


FIG.  60. 


without  alternately  inserting  and  withdraw- 
ing it,  provided  its  polarity  be  alternately 
reversed,  by  reversing  the  battery  current. 

The  production  of  electric  currents  in  this 
way  is  what  might  be  expected,  since  mag- 
netism is  induced  in  a  bar  of  iron  by  passing 
an  electric  current  around  it  on  a  helix  (772). 

782.    Magneto-electric    machines 

are  constructed  in  various  ways,  so  as  to  re- 
produce all  the  phenomena  of  statical  and 
voltaic  electricity  from  permanent  magnets. 
Such  machines  are  often  employed  in 
making  application  of  electricity  in  the  treat- 
ment of  various  diseases. 


783.  Figures  61  and  62.— Diamagnetism.— It  has  been  de- 
monstrated that  all  bodies,  solids,  liquids,  and  gases  (which  have  been 
tested),  are  subject  to  magnetic  influence. 

If  any  substance  be  suspended  between  two  opposite  powerful  poles 
of  electro-magnets,  as  seen  in  the  figures,  it  will  assume  either  the 
axial  position,  as  shown  in  Fig.  61,  or  the  equatorial  position,  shown  in 
Fig.  62. 

FIG.  61.  FIG. 


If  the  body  assume  the  axial  position,  it  shows  it  is  attracted  by  the 
poles,  and,  therefore,  it  is  said  to  be  magnetic  (615). 

If  the  body  assumes  the  equatorial  position,  it  shows  it  is  repelled 
by  the  poles,  and,  therefore,  it  is  said  to  be  diamagnetic  ;  and  the  phe- 
nomena developed  have  received  the  general  name  of  diamagnetism. 

Fluids  are  tested  by  putting  them  in  small  homoeopathic  vials. 
Pieces  of  wood,  meat,  apples,  leaves  of  trees,  and  every  sort  of  sub- 
stance, will  assume  either  the  axial  or  equatorial  position. 

The  following  list  expresses  the  order  of  some  of  the  most  common 
magnetic  substances,  viz. :  iron,  nickel,  cobalt,  manganese,  palladium, 
crown-glass,  platinum,  osmium.  The  zero  is  vacuum.  The  diattHty- 
netics  are  arranged  in  the  inverse  order,  commencing  with  the  most 
neutral :  arsenic,  ether,  alcohol,  gold,  water,  mercury,  flint-glass,  tin, 
antimony,  phosphorus,  bismuth. 


408 


ELECTRICITY. 


784-  Figure  63.    Currents  induced  by  other  currents. — 

It  has  been  shown  that  the  electricity  of  the  machine  acts  upon  bodies 
FlG  63  by  induction   (668).     And  as   it  has  been 

shown,  also,  that  a  wire  carrying  a  voltaic 
current  acts  like  a  magnet,  it  ought,  by  in- 
duction to  excite  a  current  in  another  wire 
near  it.  By  experiment  this  is  found  to  be 
the  case.  The  induced  current,  however,  is 
excited  only  when  the  battery  current  begins 
to  flow  and  when  it  ceases. 

Two  insulated  wires  are  wound  in  the 
form  of  a  helix,  so  that  they  run  side  by 
side  through  their  whole  course.  Let  NS 
represent  the  battery  wire,  and  TH  the 
other  wire. 

If  a  voltaic  current  be  passed  through  NS, 
there  is  instantly  a  current  produced  by  in- 
duction in  TH,  in  the  opposite  direction.  This  current  in  TH  ceases 
in  a  moment ;  but  when  the  battery  current  is  stopped  or  broken,  then 
there  is  a  secondary  current  produced  in  TH,  in  the  opposite  direction 
to  that  first  produced.  These  are  called  induced  currents,  or  second- 
ary currents,  but  they  are  only  momentary. 

To  facilitate  the  completion  and  interruption  of  the  battery  current 
with  great  rapidity,  one  end  of  the  conjunctive  wire  may  be  attached 
to  a  coarse  steel  file  or  rasp,  over  which  the  other  end  of  the  wire  is 
drawn,  each  notch  on  the  rasp  breaking  the  current. 

785.  Figure  64. — Induced  currents  of  different  orders. 

— By  using  spirals  of  copper  ribbon,  alternating  with  helices  of  insu- 
FIG.  64.  lated  wire,  and  arranging  them 

in  the  order  shown  in  the  figure, 
it  is  demonstrated  that  second- 
ary, or  induced  currents,  pro- 
duce other  induced  currents  of 
the  second,  third,  fourth,  and 
even  as  high  as  the  ninth  order. 
At  every  rupture  of  the  pri- 
mary, or  battery  current  (from 
OP),  the  spiral  ribbon,  E,  in- 
duces a  secondary  intense  cur- 
rent, of  opposite  name,  in  the 
helix  R,  while  the  spiral  of  rib- 
bon, H,  receives  from  R  a  quan- 


ELECTRICITY. 


409 


tity  current,  inducing  a  tertiary  intense  current  in  the  second  helix,  L, 
which  will  be  realized  by  grasping  the  handles,  U  and  Y. 

786.  The  properties  of  induced  currents  are  the  same  as 
those  of  other  electrical  currents.  They  produce  violent  shocks,  give 
sparks,  decompose  water,  salts,  etc.,  and  act  upon  magnets. 

The  longer  the  wires  the  more  powerful  the  currents. 

757.  Figure  65.— Thermo-electricity.— If  the  junction  of 
two  metals,  of  unlike  crystalline  texture   and  conducting  power,  is 
heated,  an  electric  current  will  flow 
from  one  to  the  other.     Electricity 
thus  produced  is  called  thermo-elec- 
tricity. 

Let  RS  be  a  bar  of  bismuth,  tin, 
lead,  or  zinc,  over  which  place  a  strip 
of  copper,  binding  the  ends  together 
with  solder  or  rivets.  Place  between 
them  an  astatic  needle  (764).  If  the 
junction  at  R  be  headed,  as  by  the 
spirit-lamp,  the  needle  will  be  de- 
flected in  one  direction ;  if  the  other 
junction,  S,  be  heated,  the  needle 
will  turn  in  the  opposite  direction. 

Thermo-electric  currents  are  de- 
veloped by  unequally  heating  different  parts  of  the  same  metal  (780). 

Intense  effects,  analogous  to  those  of  the  voltaic  pile,  are  obtained 
from  a  compound  thermo-electric  series,  if  half  of  the  solderings  are 
heated  and  the  other  half  cooled. 


FIG. 


788.  Figure  66.  — The  thermo- 
electric revolving  arch. — A  delicate 
reaction  between  the  magnetism  of  the 
earth  and  the  electric  current  may  be  had 
by  this  instrument.  The  arch  is  a  piece 
of  brass  wire,  nicely  adjusted  on  a  pivot  at 
the  top  of  the  support,  having  its  two  ends 
connected  to  German  silver  wire,  HS,  en- 
circling the  support.  If  the  stand  be  so 
placed  that  the  wire,  HS,  points  east  and 
west,  upon  heating  the  junction  at  the  east 
the  arch  will  rotate.  The  thermo-electric 
current  will  be  set  in  motion  from  the  Ger- 


410  ELECTRICITY. 

man  silver,  through  the  heated  junction  to  the  brass  and  through  the 
arch  to  the  German  silver.  The  faces  of  the  arch  thus  acquire  polarity, 
the  face  turned  to  the  north  exhibiting  south  polarity.  The  arch, 
therefore,  will  move  round  to  present  its  other  face  to  this  pole ;  but, 
in  so  doing,  the  other  junction  is  brought  into  the  flame,  and  the  direc- 
tion of  the  current  reversed;  this  changes  the  polarity  of  the  faces, 
and  the  arch  again  moves  on ;  thus  a  slow  but  continuous  revolution 
is  produced. 

If  the  arch  be  mounted  upon  one  of  the  arms  of  a  U-magnet,  the 
rotation  will  be  more  rapid  than  in  the  above  case. 

ORGANIC     ELECTRICITY. 

789.  Animal  electricity.— It  has  been  demonstrated  that  a 
current  of  positive  electricity  is  always  circulating  from  the  interior  to 
the  exterior  of  a  muscle.     There  is  also  an  electrical  current  from  the 
outer,  or  cutaneous,  to  the  inner,  or  mucous,  surfaces  (722). 

790.  Electrical  animals.— It  has  long  been  known  that  certain 
fishes  possess  the  power  of  communicating  an  electric  shock  to  persons 
handling  them.     The  most  remarkable  of  these  are  the  torpedo,  the 
silurus  electricus,  and  the  gymnotus.     The  electric  organs  of  these 
animals  bear  a  resemblance  to  the  voltaic  pile. 

The  most  remarkable  of  these  animals  is  the  gymnotus,  or  electrical 
eel,  found  in  abundance,  by  Humboldt,  in  South  America.  They  are 
about  five  or  six  feet  long.  Electrical  experiments  are  performed  with 
them  by  means  of  two  copper  clasps,  by  which  the  animal  is  seized 
near  the  head  and  tail.  It  is  found  that  the  part  nearest  the  head  is 
positive,  and  that  nearest  the  tail  negative.  The  shocks  received  from 
this  little  animal  are  sufficient  to  throw  a  man  upon  the  floor,  mag- 
netize needles,  produce  sparks,  kill  fish  as  if  they  were  struck  by  light- 
ning, deflect  the  galvanometer,  produce  chemical  decomposition,  heat 
small  wires  red-hot,  and  destroy  the  lives  of  large  animals,  even  horses 
and  mules,  when  attacked  by  them  in  their  native  waters. 

791.  Electricity  of  plants.— It  is  estimated  that  a  surface  of 
100  square  yards  covered  with  vegetation,  disengages,  in  a  day,  more 
negative  electricity  than  is  required  to  charge  the  most  powerful  Ley- 
den  battery. 


SOLAR     SYSTEM. 
FlG.  1. 


ASTRONOMY.  413 


CHAPTEE    XVII. 

(CHART  NO.  9.) 

ASTRONOMY. 
Definitions,  Introductory  Observations,  and  Theories. 

792.  Astronomy  (signifying  the  laws  of  the  stars)  is  that  branch 
of  Physics  or  Natural  Philosophy  which  treats  of  the  heavenly  bodies 
—the  Sun,  Planets,  Satellites,  Comets,  and  Fixed  Stars. 

793.  The  general  divisions  of  the  subject  are— 

1st.  Descriptive  astronomy,  which  treats  of  the  magnitudes,  dis- 
tances, and  densities  of  the  heavenly  bodies,  and  the  phenomena  de- 
pendent on  their  motions,  such  as  day  and  night,  the  seasons,  eclipses, 
etc. 

3d.  Physical  astronomy,  which  treats  of  the  causes  of  planetary 
motion,  and  the  laws  by  which  the  movements  of  the  heavenly  bodies 
are  regulated  and  maintained. 

3d.  Practical  astronomy,  which  explains  the  construction  and  use 
of  astronomical  instruments,  and  the  application  of  astronomical 
calculations. 


794'  Different  classes  of  heavenly  bodies.  —  The  heavenly 
bodies  are  divided  into  three  classes  or  systems,  viz.,  the  solar  system, 
infixed  stars,  and  comets. 

795.  Extent  of  space.  —  There  are  no  bounds  to  space.    It  is 
illimitable.     If   we  imagine   an   indefinite   number.  of  objects,  as  nr- 
rows,  to  start  from  any  point  in  space,  and  to  fly,  in  straight  lines,  in 
different  and  opposite  directions,  with  the  speed  of  light  or  lightning, 
for  billions  of  billions  of  years,  they  would  then  be  no  nearer  to  any 
bounds  of  space  than  before  they  started. 

796.  Magnitude  of  heavenly  bodies.  —  Our  vision  being  so 
limited,  and  the  mind  so  familiar  with  objects  of  small  magnitude, 
it  is  hardly  possible  to  appreciate  the  real  magnitude  of  even  the  earth 
on  which  we  live.    Yet  the  great  green  earth   is,  relatively,  but  an 
almost  invisible  speck  within  the  boundless  empire  of  Omnipotence. 


414  ASTRONOMY. 

Our  own  sun,  which  is  but  one  of  the  minor  countless  stars,  is  a  mil- 
lion and  four  hundred  thousand  times  larger  than  the  earth. 

757.  The  number  of  the  heavenly  bodies. — It  is  estimated 
that  one  hundred  millions  of  stars  are  visible  through  the  telescope, 
which  cannot  be  discovered  with  the  naked  eye.  Yet  all  these,  we  may 
believe,  are  no  more  than  a  drop  of  water  to  the  ocean,  compared  to  the 
countless  suns  and  systems  of  worlds  that  mo.ve  in  unmeasured  orbits 
beyond  the  utmost  reach  of  the  telescope. 

798.  Distances  between  heavenly  bodies. — Our  notions  of 
distance  are  so  far  influenced  by  the  limited  spaces  over  which  we 
travel,  it  is  no  easy  task  of  the  mind  to  appreciate  even  240,000  miles, 
the  distance  between  the  earth  and  the  moon.     The  nearest  fixed  star, 
Sirius,  is  more  than  20,000,000,000,000,  or  twenty  millions  of  millions 
of  miles  from  the  earth.     Yet  it  is  believed,  and  partially  proved,  that 
other  stars  are  five  hundred  times  this  distance  from  the  earth.     Light, 
travelling  at  the  rate  of  192,000  miles  per  second,  would  require  170 
years  to  reach  the  earth  from  some  of  the  stars  of  the  sixth  magnitude ; 
while  Herschel  says  that  light  would  be  millions  of  years  in  coming 
from  some  of  the  stars  seen  through  his  40-feet  telescope. 

799.  The  orbital  motions  of  heavenly  bodies.— All  the 

heavenly  bodies  embraced  in  the  solar  system  are  in  motion.  Not  only 
do  the  satellites  move  around  the  planets,  but  the  planets  move  around 
the  sun,  and  the  sun  moves  around  some  other  body  as  its  centre.  And 
it  is  believed,  and  in  some  cases  demonstrated,  that  all  the  so-called 
fixed  stars  revolve  around  other  centres,  each  carrying  with  it  a  system 
of  planets  and  satellites;  and  the  central  sun  of  these  suns,  around 
other  orbs  ;  and  so  on. 

The  extent  of  these  orbital  movements  varies,  of  course,  with  differ- 
ent bodies.  The  distance  of  the  sun's  remotest  planet,  Neptune,  is 
2,862,000,000  of  miles;  while  the  distance  of  the  sun  from  its  own 
central  orb  is  so  great,  that  it  requires  18,000,000  of  years  to  complete 
one  revolution,  though  it  travels  at  the  rate  of  20,000  miles  per  hour. 

800.  The  velocity  of  heavenly  bodies. — The  velocity  of 
heavenly  bodies  is  inconceivable.     Mercury,  the  swiftest  of  our  planets, 
flies  in  its  orbit  at  the  rate  of  about  100,000  miles  per  hour;  and  the 
earth  about  68,000  miles  per  hour.      The  sun,  carrying  with  itself 
thousands  of  comets,  and  all  her  planets  and  their  satellites,  travels 
around  its  central  body,  as  above  stated,  at  the  rate  of  20,000  miles  per 
hour ;  while  some  of  her  comets,  in  some  parts  of  their  orbits,  fly  with 
the  amazing  swiftness  of  a  million  miles  an  hour. 


ASTRONOMY.  415 

Notwithstanding  the  vast  magnitude  and  number  of  the  heavenly 
bodies,  and  the  immense  extent  of  their  orbits,  and  the  inconceivable 
velocity  of  their  movements,  yet  there  is  room  for  them  all ;  and  in 
their  midst  is  the  Great  Unseen  Hand  that  guides  them. 

801.  Early  observations  of  astronomical  phenomena. — 
Observations  of  important    astronomical  facts   and   phenomena  were 
made   at   an   early  date   by  the   Egyptians,  Chaldeans,  Indians,  and 
Chinese,  who  possessed  many  rules  and  methods  of  astronomical  calcu- 
lations. 

The  oldest  recorded  observations  are  those  of  the  Chinese.  Their 
annals  contain  an  account  of  a  conjunction  of  five  planets  at  the  same 
time,  which  occurred  one  hundred  years  before  the  flood.  The  truth 
of  this  account  is  confirmed  by  mathematical  calculation. 

The  Greeks  doubtless  derived  much  of  their  knowledge  of  this 
science  from  Egypt. 

The  first  of  the  Greek  philosophers  who  taught  astronomy  was 
Thales,  of  Miletus,  about  640  years  before  the  Christian  era.  Then 
followed  Anaximandar,  Anaximenes,  Pythagoras,  and  Plato.  Some  of 
the  views  of  these  philosophers  were  correct,  but  they  failed  to  pro- 
duce a  connected  and  complete  system,  and  were  unable  to  demon- 
strate their  hypotheses. 

802.  Ptolemy's  Great  System. — Having  collected  the  opin- 
ions of  all  antiquity,  and  those  of  the  philosophers  that  preceded  him, 
Ptolemy,  an  Egyptian  philosopher,  composed  a  work  of  thirteen  books, 
called  the  Great  System. 

Though  Pythagoras  taught  that  the  Sun  was  the  centre  of  the  uni- 
verse, and  that  the  earth  had  a  diurnal  motion  on  its  axis,  and  an 
annual  motion  around  the  Sun,  yet  Ptolemy,  who  flourished  130  years 
after  Christ,  rejected  these  teachings  of  Pythagoras,  as  contrary  to  the 
evidence  of  the  senses,  and  endeavored  to  explain  the  celestial  phenom- 
ena by  supposing  the  earth  to  be  the  centre  of  the  universe,  and  all 
the  heavenly  bodies  to  revolve  around  it ;  that  the  earth  was  a  plane, 
instead  of  a  globe ;  and  that  it  was  inhabited  only  on  one  side;  that 
the  stars  were  supported  in  their  places  by  being  set  or  fastened  into 
arches  or  hollow  spheres,  etc. 

In  explaining  the  celestial  phenomena,  however,  upon  his  hypothesis, 
lie  met  with  a  difficulty  in  the  apparently  stationary  attitude  and  retro- 
grade motions  which  lie  saw  the  planets  sometimes  have.  To  explain 
this,  however,  he  supposed  the  planets  to  revolve  in  small  circles,  which 
lie  called  epicycles,  which  were,  at  the  same  time,  carried  around  the 
earth  in  larger  circles,  which  he  called  deferents,  or  carrying  circles. 


416  ASTRONOMY. 

803.  Copernicus'  theory. — About  the  middle  of  the  15th 
century,  Copernicus,  a  native  of  Prussia,  having  an  intense  passion  for 
the  pursuit  of  astronomy,  quitted  the  profession  of  medicine  and  turned 
his  attention  to  this  science.  He  conceived  the  idea  that  simplicity 
and  harmony  should  characterize  the  arrangements  of  the  planetary 
system.  In  the  complication  and  disorder  which  he  saw  in  the 
hypothesis  of  Ptolemy,  he  perceived  insuperable  objections  to  its  being 
considered  as  a  representation  of  nature. 

In  the  opinions  of  the  Egyptian  sages,  and  those  of  Pythagoras  and 
others,  Copernicus  recognized  his  own  earliest  convictions,  that  the 
earth  was  not  the  centre  of  the  universe.  By  laboring  more  than  thirty 
years,  in  clearing  away  various  hypotheses,  and  gradually  expelling  the 
difficulties  with  which  the  subject  was  encumbered,  he  was  permitted 
to  see  the  true  system  of  the  universe. 

The  sun  he  considered  as  immovable,  in  the  centre  of  the  system, 
while  the  earth  revolved  around  it,  between  the  orbits  of  Venus  and 
Mars,  and  produced,  by  its  rotation  about  its  axis,  all  the  diurnal 
phenomena  of  the  celestial  sphere.  The  other  planets  he  considered 
as  revolving  about  the  sun,  in  orbits  exterior  to  that  of  the  earth. 

804-  Kepler's  discoveries  and  laws. — At  the  close  of  the 
15th  century,  Kepler,  a  German,  discovered  and  proved  that  the  orbits 
of  the  planets,  and  those  of  their  moons,  were  not  circular,  but  elliptical. 
The  supposition  that  they  were  circular  had  caused  much  error.  He 
next  determined  the  dimensions  of  the  orbits  of  the  planets,  and  found 
to  what  their  velocities,  and  their  motions  through  their  orbits,  arid 
the  times  of  their  revolutions,  were  proportioned  ;  which  are  truths  of 
the  greatest  importance  to  the  science. 

The  three  great  laws  of  Kepler  are : 

1st.  That  all  the  planets  revolve  in  elliptical  orbits,  having  the  sun  in 
one  of  their  foci. 

3d.  That  the  radius  vector  passes  over  equal  areas  in  equal  portions 
of  time. 

3d.  That  the  square  of  the  times  of  the  revolutions  of  the  planets 
around  the  sun,  are  proportional  to  the  cubes  of  their  mean  distances 
from,  the  sun. 

805.  Galileo's  discoveries. — While  Kepler  was  discovering  and 
demonstrating  the  above  important  laws,  Galileo,  an  Italian,  having 
improved  the  telescope,  was  discovering  mountains  and  valleys  upon 
the  surface  of  the  moon ;  satellites  or  secondaries  were  discovered  re- 
volving about  Jupiter ;  and  Venus,  as  had  been  predicted  by  Coper- 
nicus, was  seen  exhibiting  all  the  different  phases  of  the  moon. 


ASTRONOMY.  417 

All  these  discoveries  and  many  others  served  to  confirm  the  Coper- 
nican  theory,  and  to  show  the  absurdity  of  the  hypothesis  of  Ptolemy. 

806.  Newton's  discovery. — Notwithstanding  the   important 
discoveries  of  Copernicus,  Kepler,  and  Galileo,  the  force  which  causes 
the  planets  to  revolve  around  in  their  orbits  was  yet  unknown.     To 
ascertain  the  cause  of  the  planetary  motions,  and  explain  the  laws  by 
which  these  vast  orbs,  in  their  rapid  flight,  are  directed,  each  in  its 
own  definite  course,  constituted  the  discovery  of  the  illustrious  Newton. 
He  conceived  the  idea,  that  the  same  force  which  causes  apples  to  fall 
from  a  tree  might  extend  to  the  moon,  and  hold  it  in  its  orbit,  and 
cause  it  to  revolve  around  the  earth.     By  a  series  of  calculations  he 
established  the  fact,  that  the  same  force  which  causes  a  pebble  to  fall 
from  the  hand  to  the  ground,  carries  the  moons  in  their  orbits  around 
the  planets,  and  the  planets  and  comets  in  their  orbits  around  the  sun. 
This  force  is  the  power  of  attraction. 

THE     SOLAR    SYSTEM. 

Classification. 

807.  Figure  1.— The  Solar  System   (see  frontispiece).— By 
the  Solar  System  is  meant  the  Sun  and  the  heavenly  bodies  that  revolve 
about  it,  including  the  satellites. 

Planets  (signifying  wanderers)  are  primary  or  secondary.  The 
primary  planets  are  those  which  revolve  around  the  sun  as  their  proper 
centre ;  one  of  which  is  our  earth.  The  secondary  planets  are  those 
which  revolve  around  the  primaries,  as  they  are  carried  around  the 
sun ;  one  of  which  is  our  moon. 

The  primaries  are  usually  called  planets  ;  the  secondaries  are  called 
moons,  or  satellites. 

The  planets  are  dark  opaque  bodies,  and  shine  only  by  reflecting  the 
light  of  the  sun.  They  may  be  distinguished  from  the  stars  by  their 
steady  light;  while  the  stars  appear  to  twinkle.  They  seem  to  change 
their  relative  places  in  the  heavens,  for  which  reason  they  are  called 
planets ;  while  those  luminous  bodies  which  are  called  fixed  stars  ap- 
pear to  preserve  the  same  relative  position. 

Primary  planets. — There  are  ninety-three  primary  planets; 
eighty-five  of  which  revolve  in  orbits  very  near  each  other,  situated 
between  Mars  and  Jupiter ;  and,  on  account  of  their  small  size  and 
star-like  appearance,  they  are  called  Asteroids.  Only  five  of  these  are 
represented  in  the  illustration  (Fig.  1). 

27 


418 


ASTRONOMY. 


FIG.  2. 


The  other  eight  of  the  primaries 
are,  beginning  with  the  one  nearest 
to  the  Sun,  Mercury,  Venus,  Earth, 
Mars  (Asteroids),  Jupiter,  Saturn, 
Uranus,  Neptune. 

Satellites  or  moons. — There 
have  been  discovered  twenty  second- 
aries or  satellites.  Of  these,  the 
earth  has  one,  Jupiter  four,  Saturn 
eight  (and  two  rings),  Uranus  six, 
Neptune  one. 

The  interior  and  exterior 
planets. — The  interior  planets  are 
those  whose  orbits  lie  within  the 
orbit  of  the  earth.  The  exterior 
planets  are  those  whose  orbits  lie 
without  the  orbit  of  the  earth. 

Comets  are  a  singular  class  of 
bodies,  belonging  to  the  Solar  Sys- 
tem, revolving  in  greatly  elongated 
orbits,  and  various  in  form;  some 
being  globular,  and  others  having 
long  trains  of  light.  Two  of  these, 
S  and  E,  are  represented  in  the 
illustration  (Fig.  1) ;  only  a  part  of 
the  orbit  of  R  being  shown,  while 
that  of  S  is  complete. 

By  solar  bodies  is  meant  those 
bodies  which  belong  to  the  solar 


808.    Figure    2.— Relative 
magnitudes  of  the  planets.— 

No.  1  represents  one  of  the  larger 
Asteroids ;  No.  2,  the  moon  (drawn 
much  too  large);  No.  3,  Mercury; 
No.  4,  Mars ;  No.  5,  Venus ;  No.  6, 
the  Earth ;  No.  7,  Neptune ;  No.  8, 
Uranus;  No.  9,  Saturn;  No.  10, 
Jupiter. 


ASTRONOMY. 


419 


809.  Figure  3.— Approximate  relative  distances  of  the 
planets. — Though  it  is  impossible,  as  hereafter  shown,  to  represent 
FIG.  3. 


420  ASTRONOMY. 

on  paper  the  correct  relative  distances  between  heavenly  bodies,  yet 
this  diagram  will  convey  a  less  erroneous  idea  than  Fig.  1.  It  is  drawn 
on  the  chart  to  a  scale  of  about  66,000,000  miles  to  the  inch. 

The  first  circle,  which  is  drawn  very  near  to  the  sun,  represents  the 
orbit  of  Mercury ;  the  next  beyond,  the  orbit  of  Venus ;  the  circle,  E, 
represents  the  orbit  of  the  Earth  ;  the  line,  M,  a  part  of  the  orbit  of 
Mars ;  the  lines,  A,  a  few  of  the  orbits  of  the  Asteroids ;  J,  the  orbit  of 
Jupiter ;  S,  the  orbit  of  Saturn ;  U,  the  orbit  of  Uranus ;  and  N,  the 
orbit  of  Neptune. 

The  reader  will  imagine  the  cut  on  the  right  to  be  placed  at  the 
bottom  of  the  one  on  the  left,  as  it.  is  drawn  on  the  chart,  where  the 
figure  is  four  feet  long. 

The  arrows,  in  all  cases,  represent  the  direction  of  the  motions  of  the 
various  bodies.  In  these  several  orbits  are  represented  the  primaries, 
together  with  the  satellites  and  their  orbits. 

810.  Impossibility  of  delineating  the  solar  system.— 

The  magnitude  of  heavenly  bodies  and  the  distances  between  them 
are  so  great,  and  yet  so  unequal,  that  if  the  smallest  and  nearest  are 
drawn  on  a  scale  large  enough  to  be  seen,  then  the  largest  become  so 
great  and  the  most  distant  ones  so  remote  that  they  exceed  all  possible 
extent  of  drafting  surfaces,  as  of  paper,  cloth,  etc. 

To  illustrate  (by  referring  to  the  diagram,  Fig.  1),  suppose  the  orbit 
of  the  earth  (third  from  the  centre)  to  be  95,000,000  of  miles  (its  real 
distance)  from  the  sun.  Now,  as  the  distance  of  Neptune,  the  most 
remote  planet,  is  2,862,000,000  of  miles  from  the  sun,  it  would  require, 
in  order  to  carry  out  the  scale,  that  the  outer  circle  of  the  diagram  be 
about  fourteen  feet  in  diameter. 

The  fixed  stars,  as  represented,  appear  to  be  situated  just  beyond  the 
solar  system,  which  conveys  a  very  erroneous  idea.  The  distance  from 
the  sun  to  Neptune  is  only  2,862,000,000  of  miles,  while  the  distance 
from  the  sun  to  the  nearest  star  is  20,000,000,000,000  of  miles.  There- 
fore, to  carry  out  the  scale,  the  nearest  of  these  stars,  in  the  drawing, 
should  be  placed  about  a  mile  and  a  third  beyond  the  orbit  of  Neptune. 

Solar  system  represented  by  real  objects. — To  assist  the 

student  to  obtain  a  more  correct  notion  of  the  relative  magnitudes  and 
distances  relating  to  the  solar  system,  than  can  be  gained  from  any 
possible  delineation,  let  him  imagine  a  globe  of  wood,  representing  the 
sun,  a  trifle  less  than  five  feet  in  diameter,  to  be  placed  upon  an  exten- 
sive plane,  as  a  field  of  ice.  Then,  place  about  it  other  globes,  of  the 
sizes  of  those  shown  in  Fig.  2,  on  the  chart,  which  represent  the  relative 
magnitudes  of  the  planets. 


ASTRONOMY.  421 

First  take  Mercury,  No.  3,  size  of  a  small  pea,  and  place  it  194  feet 
from  the  sun ;  then  Venus,  No.  5,  size  of  a  small  cherry,  and  place  it 
362  feet  from  the  sun;  next  the  Earth,  No.  6,  also  the  size  of  a  cherry, 
and  place  it  500  feet  from  the  sun ;  next  Mars,  No.  4,  size  of  a  cran- 
berry, and  place  it  762  feet  from  the  sun — omitting  the  Asteroids, 
some  of  which  would  be  about  the  size  of  pin-heads  and  others  the  size 
of  No.  1 — then  Jupiter,  No.  10,  size  of  a  small  citron,  and  place  it  2,600 
feet,  or  about  half  a  mile  from  the  sun;  next  8aturn,^o.  9,  also- the 
size  of  a  citron,  and  place  it  4,768  feet  from  the  sun;  then  Uranus, 
No.  8,  size  of  a  peach,  and  place  it  9,591  feet,  or  about  two  miles  from 
the  sun ;  and  last  of  all  Neptune,  No.  7,  also  the  size  of  a  peach,  and 
place  it  15,366  feet,  or  nearly  three  miles  from  the  sun. 

Now,  at  these  several  distances,  describe  circles  around  the  globe  of 
wood.  These  circles  will  represent  the  several  orbits  of  the  planets — 
the  orbits  themselves  being,  of  course,  only  imaginary  circles. 

Hence  it  is  seen  that,  although  Mercury  in  this  jscale  is  only  the  size 
of  a  small  pea,  yet  Neptune  is  nearly  three  miles  from  the  sun,  having 
an  orbit  of  about  six  miles  in  diameter. 

Representation  of  the  motions  of  the  planets.— To  imitate 
the  motions  of  the  planets  at  the  distances,  as  above  described,  suppose 
these  small  bodies  to  revolve  around  the  globe  of  wood  at  such  rates  of 
velocity  that  each  will  describe  its  own  diameter,  as  follows :  Mercury 
in  41  seconds ;  Venus,  in  4  minutes  14  seconds ;  the  Earth,  in  7  min- 
utes; Mars,  in  4  minutes  48  seconds;  Jupiter,  in  2  hours  56  minutes; 
Saturn,  in  3  hours  13  minutes;  Uranus,  in  12  hours  16  minutes;  and 
Neptune,  in  23  hours  25  minutes. 

The  Sun. 

811.  Influence  of  the  sun. — The  sun  is  the  centre  of  the  solar 
system,  around  which  all  other  solar  bodies  revolve,  and  by  which  they 
are  all  held  in  their  orbits.     It  is  a  vast  and  fiery  orb,  the  great  source 
of  light  and  heat  to  all  the  planets.     All  animal  and  vegetable  life  and 
growth  are  due  to  its  influence. 

812.  Magnitude  of  the  sun.— The  sun  is  by  far  the  largest  of 
the  heavenly  bodies  whose  dimensions  are  known.     Its  diameter  is 
889,000  miles,  and  its  volume  1,400,000  times  larger  than  that  of  the 
earth,  and  500  times  larger  than  all  the  other  bodies  of  the  solar  system 
put  together.     If  it  were  placed  where  the  earth  is,  it  would  extend 
203,000  miles,  on  all  sides,  beyond  the  orbit  of  the  moon.     The  weight 
of  the  sun  is  about  750  times  the  mass  of  all  the  rest  of  the  solar 
system. 


422  ASTRONOMY. 

813.  The  distance  of  the  sun  from  the  earth  is  95,000,000  of 
miles.  It  is  useless,  however,  to  attempt  to  impress  the  mind  with  any 
definite  idea  of  such  a  vast  distance.  A  ball  fired  from  a  cannon,  and 
flying  with  undiminished  velocity,  would  be  1,300  years  in  reaching 
the  sun.  Yet  it  requires  great  imagination  to  conceive  the  passage  of 
a  cannon-ball  for  1,300  years,  moving  at  the  rate  of  16  miles  per  minute, 
and  its  arrival  at  the  suii. 

814-  Telescopic  view  of  the  sun.— Dark  spots.— Viewed 
through  the  telescope,  the  sun  presents  the  appearance  of  an  enormous 
globe  of  fire,  frequently  in  a  state  of  violent  agitation.  Dark  spots,  of 
irregular  form,  frequently  pass  across  its  disk  from  east  to  west,  in  the 
period  of  nearly  fourteen  days.  Some  of  these  are  50,000  miles  in 
breadth. 

The  sun  was,  for  ages,  and  till  lately,  thought  to  be  a  globe  of  real 
fire ;  but  it  is  now  believed  to  be  an  opaque  body,  surrounded  by  a 
luminous  atmosphere. 

Motions  of  the  sun. — The  sun  has  three  motions.  1st,  It  rotates 
on  its  axis  once  in  25  days,  9  hours,  36  minutes ;  its  axis  inclining 
7^-  degrees  to  that  of  the  ecliptic  (847).  2d,  It  revolves  around  the  centre 
of  gravity  of  the  solar  system  (845).  3d,  It  revolves  around  some  other 
central  body  (893). 

The  Primary  Planets. 

815.  Periodic  revolutions. — The  planets  revolve  around  the 
sun  from  west  to  east.  The  passage  of  a  planet  from  any  point  in  its 
orbit,  around  to  the  same  point  again,  is  called  its  periodic  revolution, 
and  the  time  occupied  in  making  such  revolution  is  called  its  periodic 
time. 

The  periodic  times  of  the  planets  are  as  follows : 


Mercury 88  days. 

Venus 225     " 

Earth 1  year. 

Mars..          .   1     "      322     « 


Jupiter. . .  11  years,  317  days. 

Saturn  ...  29     "     175     " 
Uranus ...     84     " 

Neptune..  164     " 


Neptune  travels  in  one  periodic  revolution  as  far  as  a  train  of  cars, 
at  30  miles  per  hour,  would  travel  in  about  70,000  years. 

The  periodic  time  of  a  planet  is  called  its  year  ;  hence,  the  year  and 
seasons  of  Neptune  are  164  times  as  long  as  those  of  the  earth,  and 
those  of  Mercury  only  about  a  quarter  as  long  as  ours. 

816.  Velocity  of  the   planets  in  their  orbits.— The  fol- 


ASTRONOMY. 


423 


lowing  table  shows  the  distance  each  planet  moves  in  its  orbit,  per 
hour: 

Jupiter 30,000  miles. 

Saturn 22,000       « 

Uranus 15,000      " 


Mercury 95,000  miles. 

Venus 75,000       « 

Earth 68,000      « 

Mars 55,000       " 


Neptune 11,000 


It  will  be  noticed  that  the  nearer  the  planet  is  to  the  sun,  the  greater 
its  velocity,  and  the  shorter  its  periodic  time. 

817.  Diurnal  revolution  of  the  planets. — Besides  the  mo- 
tion of  the  planets  around  the  sun,  they  have  a  motion  around  their 
respective  axes,  producing  the  vicissitudes  of  day  and  night.  The 
times  of  the  revolutions,  and,  consequently,  the  length  of  days  of  the 
several  planets,  are  as  follows : 


Mercury 24    hours. 

Venus 23|       " 

Earth 24 

Mars 24|       " 


Jupiter 10    hours. 

Saturn 10J-      " 

Uranus unknown. 

Neptune unknown. 


It  will  be  observed  that  the  days  and  nights  of  Jupiter  and  Saturn 
are  only  about  five  hours  long. 

The  fact  that  the  planets  revolve  around  their  axes,  is  ascertained  by 
observing  spots  on  their  surfaces,  and  noting  the  direction  of  the  mo- 
tions of  these  spots,  and  the  times  of  their  reappearance. 

818.  Magnitude  of  the  planets.— As  previously  stated,  Fig.  2 
(808)  represents  the  relative  magnitudes  of  the  planets.  Their  absolute 
magnitudes,  expressed  by  the  length  of  their  diameters,  are  as  follows : 


Mercury 3,000  miles. 

Venus 7,700      " 

Earth 8,000      " 

Mars 4,200     « 


Jupiter 89,000  miles. 

Saturn 79,000      « 

Uranus 35,000      " 

Neptune 35,000     « 


81 9.  Relative  magnitude  of  the  planets,  the  earth  being 

taken  as  the  unit  (see  Fig.  2). 


Mercury 
Venus 
Earth 
Mars 


1. 


The  Sun 


Jupiter 1,400. 

Saturn 1,000. 

Uranus 90. 

Neptune    90. 

...1,400,000. 


424  ASTRONOMY. 

820.  The  distances  of  the  planets  from  the  sun,  expressed 
in  miles,  are  as  follows : 


Mercury 37,000,000 

Venus 69,000,000 

Earth 95,000,000 

Mars .....145,000,000 


Jupiter , .     495,000,000 

Saturn 900,000,000 

Uranus 1,800,000,000 

Neptune 2,800,000,000 


Such  are  the  vast  distances  over  which  the  sun  sends  its  genial  rays 
to  light  and  warm  and  develop  its  attendant  worlds. 


Density  of  the  planets.  —  By  density  is  meant  compact- 
ness or  closeness  of  parts.  The  weight  of  a  body,  of  given  bulk,  de- 
pends upon  its  density. 

The  relative  densities  of  the  planets,  and  the  substances  with  which 
they  most  nearly  agree  in  weight,  the  earth  being  taken  as  the  standard 
or  unit  of  comparison,  are  as  follows  : 


Mercury 3  —  lead. 

Venus ^o  —  earth. 

Earth 1 

Mars -3^5—  earth. 


Jupiter \  —  water. 

Saturn TO  ~  cork. 

Uranus J  —  water. 

Neptune unknown. 


The  value  of  this  table  is  seen  in  the  following  paragraph. 


.  Attraction  of  the  planets.  —  Attraction  or  gravitation  is 
the  force  with  which  bodies  are  drawn  toward  each  other.  The  essen- 
tial law  of  this  force  is,  that  its  intensity  is  inversely  as  the  square  of  the 
distance  between  the  bodies  (47). 

The  attractive  force  of  a  planet,  therefore,  depends  upon  its  distance, 
density,  and  bulk.  Weight  is  the  amount  of  attraction  at  the  surface 
(39)  ;  hence,  the  weight  of  a  given  body,  as  a  square  foot  of  iron,  upon 
the  surface  of  any  planet,  will  depend  upon  the  density  and  bulk  of  the 
planet. 

Assuming  some  object,  as  a  piece  of  iron,  to  weigh  on  the  earth  1 
pound,  then  its  weight  on  other  planets  will  indicate  their  power  of  at- 
traction, as  compared  with  the  earth.  It  would  weigh  on  the  several 
planets,  respectively,  as  follows  : 


Mercury 1  Ib.    1£  ozs. 

Venus 0  "    15     " 

Earth 1  « 

Mars  .  .  0  "      8     " 


Jupiter 2  Ibs.    8  ozs. 

Saturn 1    «       5J  « 

Uranus 0    "     12J  « 

Neptune unknown. 


On  the  Sun  the  same  object  would  weigh  28  Ibs.  5J  ozs. 
A  person  weighing  150  Ibs.  on  the  Earth,  would  weigh  375  Ibs.  on 
Jupiter,  and  only  75  Ibs.  on  Mars. 


ASTRONOMY.  425 

823.  Light  and  heat  of  the  planets.— The  intensity  of  solar 
light  and  heat  diminishes  as  the  square  of  the  distance  from  the  sun 
increases  ;  hence,  the  amount  of  light  and  heat  derived  from  the  sun 
by  the  several  planets  is  very  unequal. 

The  relative  intensity  of  these  two  elements  or  agents  on  the  differ- 
ent planets  (their  intensity  on  the  earth  being  taken  as  the  unit  of 
comparison),  is  as  follows : 


Mercury 6 

Venus 2 

Earth 1 

Mars 


Jupiter 
Saturn 
Uranus 
Neptune 


If  the  average  temperature  of  the  earth  is  50°  F.,  that  of  Mercury 
would  be  325°,  or  113°  above  that  of  boiling  water,  and  that  of  Neptune 
1,300  times  lower  than  the  average  of  the  earth. 

It  does  not  necessarily  follow  that  the  heat  is  proportionate  to  the 
light  received  by  the  respective  planets,  as  various  local  causes  may 
modify  the  temperature.  Mercury,  for  instance,  may  be  surrounded 
by  an  atmosphere  that  arrests  the  light  and  screens  the  planet  from  the 
intense  heat  of  the  sun ;  while  the  atmospheres  of  the  more  distant 
planets,  as  Saturn,  Uranus,  etc.,  may  act  as  a  refracting  medium,  to 
gather  and  concentrate  light  and  heat  upon  these  planets. 

The  Asteroids. 

824-  Tlie  Asteroids  (Figs.  1  and  3).— As  previously  stated  (807), 
there  are  eighty-five  small  planets,  whose  orbits  are  situated  between 
those  of  Mars  and  Jupiter,  five  of  which  are  represented  by  the  five 
lines  drawn  near  each  other  (Fig.  1).  Four  of  these,  Ceres,  Pallas, 
Juno,  and  Vesta,  were  discovered,  respectively,  in  the  years  1801,  1802, 
1804,  and  1807.  In  1845,  another,  Astraea,  was  discovered;  since  when 
they  have  been  discovered,  one  after  another,  until,  up  to  1865,  they 
number  in  all  eighty-five. 

The  asteroids  all  revolve  at  nearly  the  same  distance  from  the  sun, 
and  perform  their  periodic  revolutions  in  nearly  the  sam.e  time.  Their 
orbits  are.  more  eccentric  than  those  of  the  larger  planets,  and  some  of 
them  cross  each  other,  as  shown  in  Fig.  1.  From  these  and  other  cir- 
cumstances, it  is  believed  that  these  eighty-five  small  planets  are  the 
fragments  of  a  large  planet  which  once  revolved  between  Mars  and 
Jupiter,  and  which,  by  some  convulsion  or  violence,  was  burst  asunder. 

Vesta  appears  like  a  star  of  the  sixth  magnitude,  and  is  the  only 
asteroid  that  can  be  seen  with  the  naked  eye. 

The  diameter  of  Ceres  is  1,585  miles ;  that  of  Pallas  2,025  miles. 


426 


The  following  table  comprises 
times  of  the  Asteroids. 


ASTRONOMY. 

the  names,  distances,  and  periodic 


No.       Names. 

Distance    from 
the  sun  in 
Miles. 

Period- 
ic  time 
in  Day?. 

No.       Names. 

Distance    from 
the  sun  in 
Miles. 

Period- 
ic  time 
in  Days. 

1.  Ceres  

262,764,110 

1,680 

44.  Nysa  .... 

230,886  670 

1  384 

2   Pallas  . 

263,186,670 

1684 

45   Eugen  ia 

260  568  660 

1  659 

3.  Juno  . 

253,524,410 

1,592 

46.  Hestia 

241  296  960 

1  479 

4.  Vesta  

224,327,205 

1,325 

47.  Aglaia  . 

273,641  325 

1  786 

5.  Astrsea  

244,767,500 

1,511 

48.  Doris  

295,150,275 

2000 

6.  Hebe  

230,414,710 

1  380 

49.  Pales     . 

293  180  925 

1  980 

7.  Iris  

226,683,965 

1,346 

50.  Virginia  ... 

251,844430 

1  577 

8  Flora  ...     . 

209,131,670 

1  193 

51.  Nemausa. 

225  901  640 

1  339 

9.  Metis  

226,644,350 

1346 

52.  Europa 

294  330  710 

1  992 

10.  Hygeia  

299,190,435 

2041 

53.  Calypso  

248,224,930 

1,543 

11.  Parthenope  

232,995,860 

1403 

54.  Alexandra  

258,811,540 

1,642 

12.  Clio  

221,617,045 

1301 

55.  Pandora 

263  965  195 

1  692 

13.  Egeria  

244,684,375 

1^510 

56.  Melete  

245,428,700 

1  517 

14.  Irene  

245,989,960 

1,522 

57.  Mnemosyne  .... 

299,942,265 

2,049 

15.  Eunomia  
16.  Psyche  

251,197,100 
277,661,440 

1,570 
1825 

58.  Concordia  .... 
59.  Olympia  

255,971,895 
257,714,955 

1,615 
1632 

17   Thetis 

235,002,450 

1421 

60.  Echo 

227  203  995 

1  351 

18.  Melpomena 

218,125,700 

1  271 

61.  Danae  .    . 

285  377,815 

1902 

19.  Fortuna.  .  .  . 

231,929,960 

1*393 

62.  Erato  

297,430,750 

2024 

20.  Massilia  

228,891,670 

i'366 

63.  Ansonia  

227,654,200 

1,355 

21   Lutetia 

231,365,945 

1388 

64   Angelina. 

254,437,170 

1  601 

22.  Calliope  

237,080,005 

l'440 

65.  Cybele  

325,996,965 

2322 

23   Thalia 

249,738,280 

1557 

66   Maja 

252  117  278 

1  579 

24.  Themis 

299,244,965 

2*042 

67.  Aria  .    . 

229  421  200 

1  371 

25.  Phoccea.. 

228,100,700 

1*359 

68.  Leto  

258,652,510 

1641 

26.  Proserpine  

252,327,505 

1581 

69.  Hesperia  

290,924,010 

1,957 

27   Euterpe 

222,993,975 

1  314 

70.  Panopasa  .    . 

253  662,065 

1594 

28   Bellona  

263,641,815 

1689 

71.  Feronia  

203,783,740 

1  148 

29.  Amphitrite  .... 

242,712,270 

l'492 

72.  Niobe  

261,841,470 

1,671 

30    Urania 

224  598,905 

1  328 

73.  Clytie    .  . 

254  435  102 

1,589 

31.  Euphrosyne 

299  835,010 

2?048 

74.  Galatea  

244,645,135 

1,509 

32.  Pomona.  ....... 

245,958,705 

1,522 

75.  Euridice  

251,121,955 

1,570 

33   Polymnia 

272,372  125 

1  773 

76.  Freia     . 

302  955  000 

2,080 

34.  Circe  ...     . 

255,388,690 

1610 

77.  Frigga  

253,521,413 

1,597 

35.  Leucothea  

288,216,755 

1,880 

78.  Diana  

262,418,500 

1,677 

36   Atalanta 

261  126  975 

1  665 

79.  Eurynome 

232  294  000 

1,397 

37   Fides 

255,981,165 

1,568 

80.  Sappho  

215  390  742 

1,271 

38.  Leda  

260,270,075 

1,656 

81.  Terpsichore.  .  .  . 

263,981,794 

1,693 

39    Leetitia 

263  091,765 

1  683 

82.  Alemene 

257  814  930 

1,659 

215,379,060 

1,247 

83.  Beatrix  

232  297  428 

1,382 

41    Daphne 

228  032  015 

1  358 

84   Clio 

225  900  271 

1  324 

42   Isis 

231  219  455 

1,387 

85.  lo  

252  117  294 

1572 

43.  Ariadne  

209,364,610 

1,195 

The  Secondary  Planets  or  Satellites. 

825.  Compound  motion  of  the  satellites.— The  relative 
magnitudes  and  distances  of  the  satellites  are  not  shown  in  Figs.  1  and 
3,  though  their  approximate  relative  distances  are  represented  by  Fig. 
11  (853),  which  will  be  referred  to  again. 

As  the  primaries  revolve  around  the  sun,  so  the  satellites  revolve 
around  their  primaries.  Like  the  primaries,  they  all  revolve  from 


ASTRONOMY. 


427 


west  to  east,  except  those  of  Uranus  and  Neptune,  which  revolve  from 
east  to  west,  as  indicated  by  the  arrow  in  Fig.  1. 

Satellites  not  only  revolve  around  the  primaries,  but  accompany  these 
in  their  journeys  around  the  sun,  besides  revolving  around  their  own 
axes ;  hence  they  have  a  compound  motion.  The  actual  track,  there- 
fore, which  a  satellite  pursues  through  space  is  by  no  means  a  simple 
curve,  as  will  be  seen  by  observing  the  track  of  the  Moon,  as  repre- 
sented in  Figs.  13,  14,  and  15  (855,  859,  and  860),  to  be  explained 
hereafter. 

Like  the  primaries,  the  satellites  receive  their  light  and  heat  from 
the  sun.  They  serve,  in  the  economy  of  nature,  to  reflect  the  light  of 
the  sun  upon  their  primaries ;  thus  diminishing  the  darkness  of  their 
shadows  or  nights. 

The  following  tables  show  the  magnitudes,  distances,  and  periodic 
times  of  the  several  secondaries. 

826.  The  Earth's  satellite  or  Moon.— The  diameter  of  the 
moon  is  2,162  miles;  its  mean  distance  from  the  earth  is  240,000  miles; 
its  revolution  on  its  axis,  called  synodic  revolution,  takes  place  once  in 
29  days  12  hours  44  minutes  3  seconds ;  its  periodic  or  sidereal  revolu- 
tion is  accomplished  in  27  days  7  hours  43  minutes  11£  seconds.     The 
moon  will  be  more  particularly  described  hereafter.     See  paragraphs 
855  to  864. 

827.  Jupiter's  satellites  (Figs.  1  and  3).— The  following  table 
exhibits  the  magnitudes,  distances,  and  periodic  times  of  Jupiter's 
satellites. 


DIAMETERS 

IN   MILES. 

DISTANCES. 

PERIODIC   TIMES. 

1st  

2,500 

280,000 

1  day  19  hours. 

2d  .... 

2,200 

440,000 

8     "    12       " 

3d  .... 

3.500 

700,000 

7    "    14       " 

4th  .... 

2,890 

1,200,000 

6     «    16       « 

828.  Saturn's  satellites  (Figs.  1,  3,  and  9).— The  distances  and 
periodic  times  of  Saturn's  satellites  are  as  follows : 

Distances.  Periodic  Times.  Distances.  Periodic  Times. 

1st  . .  118,000—  22J  hours.  5th . .     336,000—  4  days  12  hours. 

2d  ..152,000— 1  day    9        "  6th..     778,000—15     «     22      " 

3d  ..188,000—1    "    21         "  7th..     940,000—22     "      0     « 

4th.. 240,000— 2    «    17        "  8th ..  2,268,000— 76     «      7      " 


428  ASTRONOMY. 

829.  Uranus'  satellites  (Figs.  1  and  3). — The  distances  and 
the  periodic  times  of  the  satellites  of  Uranus  are  as  follows : 


Distances,  Periodic  Times. 

1st  .  .120,000— 2  days  12  hours. 
3d  ..171,000—4     "      3       " 
3d  ..288,000—8     «     17       " 


Distances.  Periodic  Times. 

4th  . .   380,000—  13  days  11  hours. 
5th  .  .   777,000—  38     «      2     « 


6th  .  1,556,000—107     "     16 


These  satellites  move  from  east  to  west,  as  before  stated ;  hence  their 
motion  is  said  to  be  retrograde. 

830.  Neptune's  satellites  (Figs.  1  and  3). — So  far  as  known, 
Neptune  is  attended  by  only  one  satellite.     It  revolves  around  its  pri- 
mary in  5  days  21  hours,  at  a  distance  of  236,000  miles.     Its  motion  is 
retrograde,  that  is,  from  east  to  west,  same  as  the  satellites  of  Uranus. 

Comets — their  Nature,  Orbits,  Motions,  etc. 

83 1.  Nature  and  appearance  of  comets  (Fig.  1). — Comets 
are  bodies  which  revolve  around  the  sun.     They  are  distinguished  from 
the  planets  and  other  heavenly  bodies  by  a  luminous  tail,  which  is 
usually  on  the  opposite  side  from  the  sun ;  though  some  are  destitute 
of  this  appendage,  while  others  have  several,  spreading  out  like  a  fan. 
It  is  generally  believed  that  comets  are  nothing  but  a  mass  of  vapor, 
more  or  less  condensed  at  the  centre.     Some  are  transparent  through- 
out their  whole  extent,  and  not  sufficiently  dense  to  obstruct  the  view 
of  stars  in  their  range,  while  others  have  an  opaque  and  solid  nucleus, 
called  the  head,  as  represented  at  E  and  S,  Fig.  1.     The  head  is  some- 
times surrounded  by  an  envelope,  which  has  a  cloudy  or  hairy  appear- 
ance.     Others   seem  to  be  only  globular  masses  of  vapor.      Comets 
assume  a  great  variety  of  shapes.     Probably  most  of  them  are  only 
gaseous.     In  short,  very  little  is  known  of  the  physical  nature  of  comets. 

There  is  so  little  density  to  comets,  it  is  doubtful  if  one  would  do 
much  harm  were  it  to  come  in  collision  with  the  earth;  while  it  has 
been  mathematically  demonstrated  that  the  chances  of  such  an  event 
occurring  is  only  as  1  to  281,000,000. 

83 2.  Orbits  of  comets  (Fig.  1). — The  orbits  of  comets  are  gen- 
erally very  eccentric,  as  shown  by  the  diagram.     Some  comets  fly  many 
billions  of  miles  beyond  the  orbit  of  Neptune,  and  then  return.    Drawn 
by  the  attraction  of  the  sun,  so  nearly  in  a  direct  line  toward  the  sun, 
through  such  vast  distances,  they  acquire  an  amazing  velocity. 

The  comet  of  1680  had  a  tail  96,000,000  of  miles  in  length.  Coming 
from  a  distance  of  13,000,000,000  of  miles,  this  comet  swept  around 


ASTRONOMY.  429 

through  its  perihelion,  within  130,000  miles  of  the  sun,  with  the  im- 
mense velocity  of  a  million  miles  per  hour  ;  subject  to  a  heat  of  the  sun 
thousands  of  times  more  intense  than  that  of  red-hot  iron. 

833.  The  periodic  times  of  comets  are  very  various ;  some 
being  limited  to  a  few  years,  while  others  extend  through  centuries. 
Up  to  the  beginning  of  the  17th  century  no  correct  notions  had  been 
entertained  in  respect  to  the  paths  of  comets,  while  now  the  elements 
of  about  137  have  been  calculated.  Of  these,  30  passed  between  the 
sun  and  the  orbit  of  Mercury ;  44,  between  the  orbits  of  Mercury  and 
Venus ;  34,  between  the  orbits  of  Venus  and  the  Earth ;  29,  between 
the  orbits  of  the  Earth  and  Jupiter. 

The  periodic  times  of  three  well-known  comets  are  as  follows: 
Encke's,  1,212  days ;  Biela's,  2,461  days ;  and  Halley's,  28,000  days. 

834-  The  number  of  comets  is  not  known.  The  number  ob- 
served since  the  Christian  era  is  650.  The  best  judges  believe  there 
are  many  thousands ;  while  M.  Arago.  by  a  certain  theory,  estimates 
them  by  the  billions. 

835.  The  direction  of  the  motions  of  comets  is  not  uni- 
form, like  the  planets.     They  observe  no  one  direction,  as  from  west  to 
east.     They  move  in  every  possible  direction.     Some  move  from  west 
to  east,  others  from  east  to  west,  while  others  seem  to  come  up  from 
the  immeasurable  depths  below  the  ecliptic.     Others  appear  to  come 
down  from  the  zenith  of  the  universe ;  while  others  come  and  go  in 
every  possible  direction,  seeming   to  dash  through  space  and  whirl 
around  the  sun  promiscuously.     Yet,  of  the  hundred  or  more  whose 
elements  have  been  calculated,  49  move  from  east  to  west,  and  49  from 
west  to  east. 

Telescopic  Views  of  the  Primaries. 

836.  A  few  particulars  relating  to  the  telescopic  views 
of  the  primaries.— They  all  have  the  same  general  figure,  that  is, 
spherical  or  spheroidal,  Fig.  7  (850).     They  all  seem  to  be  surrounded 
by  an  atmosphere  of  greater  or  less  density.     Spots  and  belts  seen  upon 
their  surfaces  seem  to  be  permanent,  and  indicative  of  divisions  of  land 
and  water,  like  the  seas  and  continents  of  the  earth. 

Of  Mercury  but  little  can  be  seen,  owing  to  its  obscurity,  caused  by 
its  nearness  to  the  sun.  It  is  claimed,  however,  that  spots  and  moun- 
tains have  been  seen  upon  its  surface.  It  has  a  faint  bluish  tint. 

The  surface  of  Venus  is  variegated  with  mountains ;  some  of  which 
are  estimated  to  be  twenty  miles  high.  The  spots  vary  in  form  and 


430  ASTRONOMY. 

number.  The  atmosphere  of  this  planet  is  supposed  to  be  very  dense, 
but  only  about  three  miles  deep.  Its  color  is  silvery  white. 

If  the  Earth  were  viewed  with  a  telescope,  say  from  Mercury,  the 
continents  and  islands  would  appear  brighter  than  the  rest  of  the  sur- 
face, while  the  oceans,  seas,  and  lakes,  reflecting  less  light,  would  appear 
less  bright.  As  the  earth  revolves  on  her  axis,  these  different  shades  of 
light  or  spots  would  be  seen  crossing  the  earth's  disk  in  twelve  hours ; 
while  clouds  and  snows  would  cause  changes  in  its  appearance,  and 
show  that  the  earth  is  surrounded  with  an  atmosphere. 

The  surface  of  Mars  is  variegated  with  oceans,  seas,  continents, 
mountains,  and  vales,  which  are  discerned  with  perfect  distinctness  and 
outlines.  The  color  of  this  planet  is  red,  which  is  supposed  to  be  the 
result  of  a  dense  atmosphere. 

The  Asteroids  are  so  distant  and  small,  that  little  is  known  regard- 
ing their  appearance.  Seen  through  a  telescope,  they  have  a  pale  ash 
color. 

The  axis  of  Jupiter  has  so  little  inclination  to  the  plane  of  its  orbit, 
there  can  be  but  little  or  no  change  of  seasons  at  the  same  parallels  of 
latitudes,  nor  any  difference  in  the  length  of  its  days  and  nights. 
Hence,  there  is  perpetual  summer  in  the  equatorial  regions,  and  per- 
petual winter  in  the  polar  regions.  Viewed  through  a  telescope,  Ju- 
piter appears  to  be  surrounded  by  a  number  of  luminous  zones,  usually 
called  belts.  These  are  parallel  to  the  equator  and  to  each  other,  but 
subject  to  considerable  variation,  both  in  breadth  and  numbers. 

The  surface  of  Saturn,  like  that  of  Jupiter,  is  diversified  with  belts 
and  dark  spots.  That  which  distinguishes  this  planet  from  every 
other,  and  which  renders  it,  of  all  others,  the  most  interesting  solar 
body,  is  a  magnificent  zone  or  ring,  surrounding  the  planet.  This 
peculiarity,  the  only  one  within  the  reach  of  telescopic  observation, 
will  be  referred  to  again. 

Uranus,  through  a  telescope,  exhibits  a  small,  round,  uniformly- 
illuminated  disk,  without  rings,  belts,  or  spots. 

Neptune  is  too  far  away  from  the  earth  to  present  any  striking  pecu- 
liarities. 

Orbits,  Eccentricity  of  Orbits,  etc. 

837.  Figure  4.— Orbits  are  elliptical.— The  orbits  of  heaven- 
ly bodies  are  not  circular,  but  elliptical ;  and  the  central  body  around 
which  another  revolves,  is  always  situated  in  one  of  the  two  foci  of  the 
ellipse.  The  revolving  body,  therefore,  is  sometimes  nearer  to  the  central 
body  than  at  others.  For  example,  the  body,  S,  which  may  represent 
the  earth,  is  nearer  to  the  sun  than  when  it  is  at  T.  The  orbits  of  some 


ASTRONOMY.  431 

bodies  are  more  elliptical  than  others ;  those  of  comets  being  the  most 
so  of  any. 

838.  The  eccentricity  of  a  planet's  orbit  is  the  distance 
of  its  centre  from  the  centre  of  the  sun.     For  example,  the  dotted  line 

FIG.  4. 


(Fig.  4)  passes  through  the  centre  of  the  orbit,  and  the  distance  from 
the  centre  of  this  line  to  the  centre  of  the  sun  below  it,  is  the  eccentrici- 
ty of  the  orbit. 

The  eccentricity  of  the  orbits  of  the  several  planets,  expressed  in 
miles,  is  as  follows : 


Mercury 7,000,000 

Venus 492,000 

Earth 1,618,000 

Mars 13,500,000 

Vesta 21,000,000 

Juno 64,000,000 


Ceres 21,000,000 

Pallas 64,250,000 

Jupiter 24,000,000 

Saturn 49,000,000 

Uranus 85,000,000 

Neptune unknown. 


Although  these  distances  seem  very  great,  yet  the  orbits  do  not  de- 
viate so  much  from  a  circle  as  might  be  imagined  at  first  thought. 
For  instance,  the  mean  distance  of  the  earth  from  the  sun  is  95,000,000 
of  miles ;  hence,  its  eccentricity,  being  only  1,618,000  miles,  would 
hardly  be  noticeable. 


432  ASTRONOMY. 

839.  Aphelion  and  perihelion.— Aphelion  is  that  point  in  the 
orbit  of  a  planet  which  is  at  the  greatest  distance  from  the  sun  ;  and 
perihelion  is  that  point  in  the  orbit  which  is  nearest  the  sun. 

840.  The  radius  vector  is  a  line  drawn  from  the  sun  to  a 
planet  in  any  part  of  the  orbit,  as  the  lines  A,  E,  F,  Fig.  4. 

841-  The  radius  vector  passes  over  equal  areas  in  equal 
portions  of  time.— That  is,  if  the  areas  (Fig.  4)  1,  2,  3,  4,  5,  and  6, 
are  all  equal  to  each  other,  then  the  planet,  S,  will  pass  from  S  to  A  in 
the  same  time  that  it  would  from  A  to  E,  and  from  E  to  F,  and  from 
F  to  H,  and  so  on.  If,  then,  the  ellipse  be  divided  into  twelve  equal 
areas,  answering  to  the  twelve  months,  the  earth  will  pass  through  an 
equal  area  every  month,  but  the  space  through  which  it  passes  in  its 
orbit  will  be  decreased  during  every  month  from  the  perihelion  (at  S) 
to  the  aphelion  (at  T),  and  increased  during  every  month  from  the 
aphelion  (at  T)  to  the  perihelion  (at  8). 

84%-  Figure  5. — Circular  or  curvilinear  motion. — It  has 

been  shown  (58),  that  when  a  body  is  acted  on  by  two  forces  perpen- 

FIG.  5. 


dicular  to  each  other,  its  motion  will  be  in  a  diagonal  direction  between 
the  directions  of  the  two  forces. 

Let  S  represent  the  sun,  and  E,  the  earth.     Draw  the  line,  E,  and  the 


ASTRONOMY.  433 

line  L,  perpendicular  to  K.  If  the  earth  were  movin'g  in  the  direction 
of  L,  with  a  velocity  that  would  carry  it  over  the  arrow,  T,  in  the 
same  time  that  the  attraction  of  the  sun  would  draw  it  over  the  arrow, 
E,  then  the  resultant  of  the  two  forces  would  carry  it  over  the  dotted 
diagonal  line  to  F.  But  the  constant  force  of  attraction  of  the  sun 
causes  the  earth  to  move  in  the  direction  of  the  curved  instead  of  the 
straight  diagonal  line.  What  is  true  in  the  passage  of  the  earth  from 
E  to  F,  is  also  true  for  every  other  part  of  its  passage  around  the  sun, 
as  will  be  understood  by  inspecting  the  diagram. 

843  -  Centripetal  and  centrifugal  forces.  —  If  the  sun  should 
cease  to  attract  the  earth,  the  earth  would  instantly  pass  off  in  a  straight 
line,  tangent  to  its  orbit.  For  instance,  if  the  sun  should  cease  its 
attraction  when  the  earth  is  at  the  point  E  (Fig.  5),  the  earth  would 
pass  off  in  the  direction  of  L.  This  tendency  to  pass  off  in  a  straight 
line  is  called  the  projectile  or  centrifugal  force.  Were  this  centrifugal 
force  to  cease,  which  it  would  do  were  the  planet  to  cease  moving  in  its 
orbit,  then  the  sun  would  draw  the  planet  to  itself  by  the  force  of  attrac- 
tion. This  force  of  attraction  to  the  centre  of  motion  is  called  the 
centripetal  or  centre-seeking  force. 


844-  Why  the  planets  do  not  fall  to  the  sun.—  From  the 

explanation  of  the  centrifugal  and  centripetal  forces  just  given,  it  will 
be  seen,  that  if  these  two  forces  were  in  exact  and  constant  equilibrium 
the  orbit  of  the  planet  would  necessarily  be  a  perfect  circle.  But  as 
these  two  forces  are  not  in  constant  equilibrium,  the  orbit  is  not  a 
circle.  Now,  as  these  two  forces  are  not  in  equilibrium,  they  must 
alternately  preponderate,  otherwise  the  planet  would  either  pass  off 
from  the  sun  or  be  drawn  to  it.  But  within  certain  limits  this  is  just 
what  takes  place. 

The  earth  (Fig.  4)  in  moving  from  S  to  T,  in  the  direction  of  the 
arrows,  is  passing  further  and  further  from  the  sun,  and  the  sun's 
attraction  is  diminishing  its  velocity  ;  and  when  the  velocity  is  so  far 
diminished  that  the  centrifugal  force  is  reduced  to  an  equilibrium 
with  the  centripetal  force,  which  takes  place  at  the  aphelion  (or  point 
T),  the  centripetal  force  begins  to  preponderate  and  increase  the  velo- 
city; and  when  it  has  so  far  increased  the  velocity  that  the  centrifugal 
force  becomes  greater  than  the  centripetal,  which  takes  place  at  the 
perihelion  (or  point  S),  then  it  begins  again  to  sweep  away  from  the 
sun;  and  thus  the  planet  continues  perpetually  to  revolve. 

845.  Centre  of  gravity  and  motion  of  the  solar  system. 

—Not  only  do  the  planets  revolve  around  the  sun,  and  the  satellites 

28 


434  ASTRONOMY. 

around  the  planets,  but  between  the  sun  and  all  the  solar  bodies  that 
revolve  around  him  there  is  mutual  attraction.  That  is,  each  body 
attracts  the  sun  just  as  much  as  the  sun  attracts  it.  In  the  same  man- 
ner, each  body  of  the  solar  system  attracts  every  other  body.  Hence, 
in  any  given  position  of  all  the  solar  bodies,  there  is  some  one  point 
which  is  the  centre  of  gravity  of  the  whole  system.  If,  in  this  given 
position,  all  the  bodies  composing  the  solar  system  were  rigidly  fas- 
tened together,  as  by  rods  or  bars,  and  the  whole  rigid  system  set  to 
revolving,  it  would  continue  to  revolve  around  this  common  imaginary 
centre  of  gravity.  Now,  as  the  quantity  of  matter  in  the  sun  is  about 
750  times  greater  than  that  of  all  the  planets  and  other  solar  bodies, 
their  whole  united  force  of  attraction  is  750  times  less  than  that  of  the 
sun.  This  common  centre  of  gravity  and  motion,  therefore,  is  not  far 
from  the  sun.  Were  all  the  other  solar  bodies  situated  in  their  orbits 
on  one  side  of  the  sun,  even  then  he  would  not  be  more  than  his  own 
diameter  from  this  common  centre  of  gravity  and  motion.  Hence  the 
sun  is  justly  considered  the  centre  of  the  system.  As  the  planets  are 
continually  changing  their  relative  positions  around  the  sun,  this  com- 
mon centre  of  gravity  and  motion  is  continually  undergoing  slight 
changes  of  position,  as  regards  the  solar  system  itself;  yet  it  moves 
around  some  other  central  system  at  the  rate  of  20,000  miles  per  hour, 
completing  its  revolution  in  18,000,000  of  years. 

8Jf6.  Planes  of  orbits. — If  a  piece  of  wire  be  bent  in  the  form 
of  a  circle,  and  paper  stretched  across  and  fastened  to  the  wire,  the  wire 
may  represent  the  orbit,  and  the  paper  the  plane  of  the  orbit.  Of  course, 
the  orbit  and  plane  are  imaginary. 

847.  Figure  6.  —  The  ecliptic.  —  Suppose  the  plane  of  the 
earth's  orbit  to  pass  through  the  centre  of  the  sun,  and  to  extend  out 
on  every  side  to  the  starry  heavens.  The  great  circle  so  made  would 
mark  the  line  of  the  ecliptic,  or  the  sun's  apparent  path  through  the 
heavens.  It  is  called  the  ecliptic,  because  eclipses  happen  when  the 
moon  is  in  or  near  this  apparent  path. 

The  axis  of  the  ecliptic  is  an  imaginary  line  passing  through  the 
centre  of  the  sun,  perpendicular  to  the  plane  of  the  earth's  orbit,  and 
the  poles  of  the  ecliptic  are  the  extremities  of  this  line. 

In  the  figure  the  dotted  line,  00,  passes  through  the  centre  of  the 
oval  which  represents  the  plane  of  the  orbit  of  the  earth,  or  the  ecliptic ; 
and  the  line  E  represents  the  axis  of  the  ecliptic. 

The  other  dotted  line  in  the  figure  passes  through  the  centre  of  the 
oval  which  represents  the  plane  of  the  equinoctial.  This  plane  passes 
through  the  centre  of  the  earth,  and  coincides  with  the  equator  of  the 


ASTRONOMY. 


435 


earth.     The  axis  of  the  equinoctial  is  represented  by  the  line  A,  which, 
of  course,  is  parallel  with  the  axis  of  the  earth. 

FIG.  6. 


848.  Obliquity  of  the  ecliptic   (Fig.  6).— The  axis  of   the 
ecliptic,  E,  and  the  axis  of  the  equinoctial,  A,  form  an  angle  of  23J 
degrees;  hence,  the  plane  of  the  ecliptic  forms  the  same  angle  with  the 
plane  of  the  earth's  equator,  or  the  equinoctial.     This  inclination  of 
the  ecliptic  to  the  equator  of  the  earth,  is  23°  28',  called  the  obliquity 
of  the  ecliptic.     This  will  be  more  clearly  shown  hereafter. 

849.  Inclination  of  the  orbits  of  the  planets  to  the 
plane  of  the  ecliptic  (Fig.  6). — The  planes  of  the  orbits  of  the 
primary  planets   all  pass  through  the  centre   of    the  sun,  and  form 
angles  with  the  ecliptic  or  plane  of  the  orbit  of  the  earth.     The  incli- 
nation of  the  orbits  to  the  plane  of  the  ecliptic  is  shown  in  the  fol- 
lowing table,  several  of   which  are   represented  by  the  ovals  in  the 
figure. 


Mercury 7  degrees. 

Venus 3£        " 

Mars 2          " 

Vesta 7          " 

Astraea 7f        " 

Juno..  ,.13  " 


Ceres 10J  degrees. 

Pallas 34£       " 

Jupiter 1J       " 

Saturn 2£       " 

Uranus f       " 

Neptune 1}       " 


It  will  be  observed  that  the  orbit  of  Pallas,  marked  P,  in  the  figure, 
forms  a  much  larger  angle  with  the  ecliptic,  00,  than  any  of  the 

others. 


430  ASTRONOMY. 

850.  Figure  7.— The  figure  or  form  of  the  planets.— The 

planets,  and  heavenly  bodies  generally,  instead  of  being  exactly  round 
or  spherical,  as  usually  represented  in  the  diagrams,  are  oblate  spheroids  ; 
that  is,  their  equatorial  diameters  are  greater  than  their  polar 
diameters. 

It  is  supposed  that  the  planets  were  once  in  a  melted  or  liquid  state. 
FIG.  7.  Suppose  the  figure  to  represent 

a  planet  in  such  a  state,  and 
perfectly  round.  If,  now,  it 
begins  to  revolve  on  its  axis, 
A,  it  will  take  the  form  shown 
by  the  dotted  line. 

The  reason  of  this  is  plain. 
At  the  equator,  E,  the  centrif- 
ugal force  is  greater  than  at 
L  and  L ;  and  greater  at  L  and 
L  than  at  S  and  S ;  while  at 
the  extremities  of  the  axis  A 
it  is  nothing.  Hence,  the  rela- 
tive intensity  of  this  force,  in  different  parts  of  the  sphere,  may  be 
represented  by  the  relative  length  of  the  several  arrows;  which  show 
that  the  greatest  elongation  will  be  at  the  equator,  and  the  greatest 
contraction  at  the  poles. 

The  difference  between  the  equatorial  and  polar  diameters  of  some 
of  the  planets,  is,  respectively,  as  follows : 


Earth 26  miles. 

Mars..  .   25 


Jupiter 6,000  miles. 

Saturn 7,500       " 


Great  magnitude  and  rapid  rotation  give  Jupiter  and  Saturn  a  large 
difference  between  their  equatorial  and  polar  diameters. 

851.  Figure  8.— Venus  as  morning  and  evening  star. 

— Let  the  student  suppose  himself  to  stand  with  his  face  to  the  south, 
and  the  plane  HH  to  represent  the  visible  horizon ;  and  the  dotted 
line,  the  daily  path  of  the  sun ;  and  S,  the  apparent  position  of  the 
earth.  The  sun  is  shown  as  rising  at  E,  and  setting  at  W,  and  on  the 
meridian  at  R,  while  Venus  is  seen  revolving  around  the  sun  in  the 
direction  of  the  arrows,  from  west  to  east. 

Now,  it  is  obvious  that  when  Venus  is  at  T,  or  west  of  the  sun,  it 
passes  below  the  horizon,  or  sets,  as  at  N,  before  the  sun ;  and  rises  be- 
fore the  sun,  as  at  E.  Hence,  while  Venus  is  in  this  part  of  her  orbit, 
it  will  be  morning  star.  When  it  is  east  of  the  sun,  as  at  F,  it  will 
linger  in  the  west  after  the  sun  sets,  as  at  W,  and  is,  consequently. 
evening  star. 


ASTRONOMY. 
FIG.  8. 


437 


Figures  9  and  10.— Saturn's  rings.— Fig.  9  repre- 
sents Saturn  as  seen  through  the  telescope.  The  light  of  the  rings  is 
more  brilliant  than  that  of  the  planet.  The  rings  lie  in  the  plane  of 

FIG.  9. 


the  plan  et\  equator,  and  revolve  around  their  centre  of  motion  in  the 
same  time  that  the  planet  revolves  on  its  axis. 

As  the  axis  of  the  planet,  like  that  of  all  the  other  planets,  p reserves 
its  parallelism  in  all  parts  of  its  orbit,  the  rings,  as  seen  from  the  earth, 
will  vary  in  appearance,  as  they  are  viewed  in  different  parts  of  the 
planet's  orbit.  Sometimes  they  will  be  seen  edgewise,  when  they  will 
reflect  no  light  to  the  earth,  but  appear  like  a  dark  line  drawn  across 
the  planet.  At  other  times  they  will  be  seen  more  or  less  obliquely,  as 


438 


ASTRONOMY. 


FIG.  10.  represented  in  Fig.  9 ;  but  they  are  never 

seen,  from  the  earth,  perpendicularly,  as 
shown  in  Fig.  10.  Were  either  pole  of  the 
planet  exactly  toward  the  earth,  they  would 
then  present  a  perpendicular  view. 

As   the  planet  revolves  around  the  sun 
once  in  30  years,  of  course,  one  side  of  the 
rings  will  be  seen  during  part  of  this  pe- 
riod, and  the  other  side  during  the   other 
part  of  its  revolution. 
As  Saturn's  moons  (except  one)  revolve  around  the  planet  nearly  in 
the  plane  of  its  equator,  they  are  seldom  eclipsed. 

The  diameters  of  the  rings  and  their  distances  from  the  planet  are 
as  follows : 

Diameter  of  the  planet 79,000  miles. 

Distance  to  the  interior  ring . ; 20,000      " 

Width  of  the  interior  ring 20,000      " 

Space  between  the  two  rings 2,000      " 

Width  of  the  exterior  ring 10,000      " 

Thickness  of  the  rings 100      " 

The  diameter  of  the  outer  ring 183,000      " 

Hence,  the  thickness  is  but  y^o  part  of  the  diameter,  which,  rela- 
tively, is  thinner  than  a  sheet  of  letter-paper.  Yet  these  rings,  accord- 
ing to  Herschel,  are  composed  of  solid  opaque  matter,  and  probably  are 
inhabited. 

A  third  ring,  interior  to  those  above  described,  was  discovered  in 
1850,  by  Mr.  Bond,  of  Cambridge,  Massachusetts. 

To  the  inhabitants  of  Saturn  these  rings  appear  like  vast  arches,  or 
semicircles  of  light,  extending  from  the  eastern  to  the  western  horizon. 
During  the  daytime,  they  appear  dim,  like  a  white  cloud,  but,  as  the 
sun  goes  down,  their  brightness  increases. 

FIG.  11. 


853.  Figure  11.— Distances  of  the  satellites  from  their 
primaries. — This  figure  represents  the  distances  of  the  satellites 
from  their  primaries,  measured  in  semi-diameters  of  the  latter. 


ASTRONOMY,  439 

The  line  at  the  top  represents  the  distance  of  the  moon  from  the 
earth  as  being  60  times  as  far  as  the  distance  from  the  centre  of  the 
earth  to  its  circumference.  Each  division  of  the  line  represents  10 
semi-diameters  of  the  earth.  Taking  the  semi-diameter  as  4,000  miles, 
we  have  60  X  4,000  =  240,000- miles,  as  the  distance  of  the  moon  from 
the  earth.  The  short  cross  lines  represent  small  parts  of  the  orbits  of 
the  satellites. 

The  several  planets,  represented  in  the  figure  below  the  earth,  are 
Jupiter,  Saturn,  Uranus,  and  Neptune.  Compare  826,  827,  828,  829, 
and  830. 

854-  Figure  12. — Solar  and  sidereal  time. — The  rotation 
of  the  earth  on  its  axis  constitutes  one  of  the  most  important  elements 
in  astronomical  science ;  for  the  reason  that  it  is  taken  as  the  standard 
of  comparison  for  the  revolution  of  all  other  celestial  bodies. 

The  earth  performs  one  complete  revolution  on  its  axis  in  23  hours, 
56  minutes,  and  4.09  seconds.  This  is  called  a  sidereal  day  ;  because, 
in  that  time,  the  stars  seem  to  complete  one  revolution  around  the 
earth. 

But,  as  the  earth  advances  nearly  one  degree  eastward,  in  its  orbit, 
in  the  time  it  turns  eastward  around  its  axis,  one  rotation  will  not 
bring  the  same  meridian  around  from  the  sun  to  the  sun  again ;  there- 
fore, the  earth  must  make  somewhat  more  than  one  rotation  to  com- 
plete a  solar  day. 

Suppose  a  man  to  be  standing  at  a  given  point,  at  12  o'clock,  noon, 
on  the  earth,  at  E,  under  the  line,  SM ;  then,  when  the  earth  shall  have 
turned  on  its  axis  so  that  he  will  again  see  the  sun  in  the  meridian,  it 
will  again  be  12  o'clock,  noon.  The  earth,  in  the  meantime,  will  have 

FIG.  12. 


passed  from  E  to  F,  and  he  will  now  see  the  sun  in  the  direction  of  the 
dotted  line,  SD,  instead  of  the  direction  of  the  line,  SID,  coming  from 
the  star,  N. 

The  difference,  then,  between  the  sidereal  day  and  the  solar  day,  is 
the  length  of  time  it  takes  the  earth  to  rotate  from  the  line,  SID,  to  the 
line,  SD,  which  is  fmtr 


440  ASTRONOMY. 

If  the  earth  were  not  revolving  around  the  sun,  the  sidereal  and  solar 
days  would  be  of  the  same  length. 

In  365  solar  days  the  earth  turns  366  times  around  its  axis.  This  is 
true  of  all  planets,  whatever  be  the  length  of  their  days  or  years. 

Sidereal  days  are  always  of  the  same  length ;  but  the  solar  days  vary 
in  length  at  different  times  of  the  year.  This  variation  is  due  to  two 
causes,  namely,  the  inclination  of  the  earth's  axis  to  the  plane  of  its 
orbit,  and  the  inequality  of  its  motion  around  the  sun.  Hence,  the  time 
shown  by  a  well-regulated  clock  and  that  of  a  true  sun-dial  are  scarcely 
ever  the  same.  The  difference  between  them,  which,  sometimes,  is  16J- 
minutes,  is  called  the  Equation  of  Time,  or,  the  equation  of  solar  days. 

The  Moon — its  Path,  Phases,  etc. 

855.  Figure  13.— The  moon's  path  around  the  sun. — 

Though  the  orbit  of  the  moon  is  an  ellipse,  with  respect  to  the  earth, 
it  is  in 'reality  an  irregular  curve,  always  concave  toivard  the  sun,  as  it 
will  presently  appear. 

To  obtain  a  correct  idea  of  the  path  of  the  moon,  it  will  be  necessary 
to  consider  Figs.  14  and  15 ;  but  Fig.  13  will  be  first  explained. 

FIG.  13. 


When  the  moon  is  at  A,  and  the  earth  is  in  its  orbit  on  the  radius, 
A,  it  is  new  moon.  In  about  14  days  the  moon  reaches  F,  and  will  be 
seen  from  the  earth  as  full  moon.  In  about  14  days  more  she  reaches 
E,  when  it  is  new  moon  again  ;  and  so  on,  perpetually.  When  it  is  new 
moon,  she  is  in  conjunction  ;  when  it  is  full  moon,  she  is  in  opposition. 


ASTRONOMY.  441 

It  will  be  seen  that  the  12  revolutions  of  the  moon,  and  the  1  revo- 
lution of  the  earth  do  not  terminate  at  the  same  time.  From  the  new 
moon,  at  A,  around  to  the  new  moon,  at  Y,  are  just  12  lunar  months 
or  revolutions;  but  at  this  time  the  earth  is  19°  20'  short  of  her  start- 
ing-point, or  of  completing  her  year.  The  lunar  year,  consisting  of  12 
synodical  revolutions  of  the  moon,  or  346  days,  is  19  days  shorter  than 
the  civil  year. 

856.  Sidereal  and  synodic  revolution  of  the  moon. — For 

the  same  reason  that  a  sidereal  day  is  shorter  than  a  solar  day  (854), 
the  sidereal  revolution  of  the  moon,  which  takes  place  in  27|  days,  is 
shorter  than  the  synodic  period,  just  explained,  which  takes  place  in 
29|  days. 

857 .  The  rotation  of  the  moon  on  her  axis  takes  place  in 
the  same  time  that  she  makes  one  synodic  revolution.    Hence,  the  same 
side  of  the  moon  is  always  turned  toward  the  earth.     Therefore,  her 
day  and  night  together  cannot  occur  but  once  in  29 £  days.    The  moon, 
consequently,  has  but  one  night  and  one  day  in  her  year,  containing, 
both  together,  29  days,  12  hours,  44  minutes,  and  3  seconds. 

858.  The  moon's  libration  in  longitude  and  latitude.— 

Owing  to  the  ellipticity  of  the  moon's  orbit,  and  the  consequent  in- 
equality of  her  angular  velocity,  she  appears  to  roll  a  little  on  her  axis 
from  east  to  west,  and  west  to  east.  This  is  called  her  libration  in 
longitude. 

The  axis  of  the  moon  is  inclined  to  the  plane  of  her  orbit  only  about 
one  and  a  half  degrees ;  but  even  this  slight  inclination  enables  us  to 
see  first  one  pole  and  then  the  other,  in  her  revolution  around  the 
earth.  These  slight  rolling  motions  are  called  her  librations  in  lati- 
tude. 

FIG.  14 


859.  Figure  14.  —The  actual  path  of  the  moon  is  shown 
by  the  line,  II,  F,  A,  K.     Suppose  the  dotted  line  to  represent  a  part 


442  ASTRONOMY. 

of  the  earth's  orbit.  At  H  the  moon  crosses  the  earth's  track  240,000 
miles  behind  the  earth.  Gaining  on  the  earth,  she  passes  it  in  7  days 
at  F,  as  a  full  moon.  Continuing  to  gain  on  the  earth,  in  7  days  more, 
she  crosses  its  track  at  A,  240,000  miles  ahead.  From  this  point  the 
earth  gains  on  the  moon  for  7  days,  when  it  overtakes  her  at  P,  where 
she  is  a  new  moon.  The  earth  continues  to  gain  for  7  days  more,  when 
the  moon  will  again  cross  the  track  of  the  earth  at  K,  240,000  miles 
behind  the  earth ;  and  so  on  perpetually. 

The  motion  of  the  moon  is  never  retrograde;  that  is, 
she  never  returns  into  her  own  path  again;  for  the  reason,  that  the 
earth  moves  much  faster  in  its  orbit  than  the  moon  in  her  orbit.  In 
fact,  the  forward  motion  of  the  moon  is  never  less  than  67,500  miles 
per  hour,  for  the  reason  that  her  hourly  motion  in  her  own  orbit  is  only 
2,300  miles  per  hour,  while  that  of  the  earth  is  68,000  miles  per  hour. 

860.  Figure  15.— The  moon's  orbit  always  concave  to- 
ward the  sun. — Let  the  long  arrow  represent  the  arc  of  the  earth's 
orbit  equal  to  that  passed  through  by  the  earth  during  half  a  lunation. 

FIG.  15. 


This  arc  and  the  cord,  TS,  being  known,  it  is  found  that  the  cord,  TS, 
must  pass  more  than  240,000  miles  within  the  earth's  orbit ;  hence,  the 
moon  can  never  reach  the  cord,  as  at  E ;  therefore  the  path  of  the 
moon,  TNLS,  must  curve  toward  the  sun. 

861.  View  of  the  earth  from  the  moon. — The  appearance 
of  the  earth  to  the  inhabitants  of  the  moon  is  similar  to  the  appear- 
ance of  the  moon  to  us.  The  earth,  however,  appears  thirteen  times 
larger  than  the  moon.  Hence  it  might  be  inferred  that  a  fashionable 
trip,  with  those  inhabitants  of  the  moon  who  live  on  the  side  always 
turned  from  the  earth,  would  be  to  journey  around  where  they  could 
view  the  sublime  spectacle  of  a  heavenly  body,  apparently  ten  times 
larger  than  any  they  had  before  seen. 


ASTRONOMY.  443 

862.  Figure  16.— The  moon's  phases.— The  parallel  lines 
on  the  right  represent  rays  of  light  from  the  sun ;  the  circle  around 
the  earth,  the  orbit  of  the  moon;  A,  B,  C,  D,  F,  G,  H,  the  moon  in 
different  positions  in  her  orbit.  In  all  positions  (except  when  she  is 
eclipsed)  the  moon  is  illuminated  by  the  sun.  At  A  it  is  new  moon ; 

FIG.  16. 


in  which  position  she  is  not  visible  to  us,  as  the  unilluminated  side  is 
turned  toward  the  earth.  At  B  she  appears  like  a  crescent,  as  shown 
at  1.  At  C,  more  of  her  enlightened  side  is  visible,  when  she  appears 
to  us  like  a  half  moon  ;  which  is  i\\?  first  quarter.  At  D,  still  more  of 
the  illuminated  side  is  seen,  where  she  appears  as  represented  at  2.  At 
E,  the  enlightened  hemisphere  is  wholly  in  view,  when  she  is  said  to 
be  full  moon,  and  in  opposition.  From  E  around  to  A  again,  the  illu- 
minated side  becomes  less  and  less  visible.  At  F,  she  appears  as  seen 
at  3 ;  at  H,  as  seen  at  4.  When  seen  at  D  and  F,  the  moon  is  said  to 
be  gibbous. 

Importance  of  the  phases  and  motions  of  the  moon.— 

The  phases  and  motions  of  the  moon  afford  a  great  variety  of  interest- 
ing investigation.  From  them  the  astronomer  ascertains  the  form  of 
the  earth,  the  vicissitudes  of  the  tides,  the  causes  of  the  eclipses,  the 
distance  of  the  sun,  and,  consequently,  the  magnitude  of  the  solar  sys- 
tem, etc.  These  phenomena,  being  perfectly  obvious  to  the  unassisted 
eye,  served  as  a  standard  of  measurement  of  time  to  all  nations,  until 
the  advancement  of  science  taught  them  the  advantages  of  solar  time. 
These  phenomena  are  of  the  greatest  importance  to  the  navigator, 
guiding  him  through  the  pathless  ocean. 


444  ASTRONOMY. 

863.  Why  the  dark  side  of  the  moon  is  visible  near 
conjunction. — While  near  the  position,  A,  the  entire  disk  of  the  moon 
is  faintly  seen  by  the  naked  eye.  This  is  because  the  dark  side  of  the 
moon  is  so  much  illuminated  by  the  reflected  light  of  the  earth,  that 
the  moon  reflects  the  light  of  the  earth  back  to  the  earth  again,  as  rep- 
resented by  the  ray  of  light  S,  which  first  is  reflected  from  the  earth 
to  the  rnoon,  and  then  from  the  moon  back  to  the  earth. 

864-  Other  particulars  relating  to  the  moon. — The  size 
of  the  moon  is  one  forty '-ninth  the  size  of  the  earth. 

It  is  about  3£  times  the  weight  of  water. 

The  light  of  the  sun  is  300,000  times  greater  than  that  of  the  moon. 

The  sun  is  70,000,000  times  larger  than  the  moon;  yet  the  moon  ap- 
pears as  large  as  the  sun,  because  it  is  400  times  nearer  to  us  than  the 
sun. 

It  rises  about  50  minutes  later  every  day,  because  it  revolves  around 
the  earth  from  west  to  east ;  though  the  full  moon,  in  Sept.  and  Oct., 
rises  but  a  few  minutes  later  for  several  successive  evenings ;  owing  to 
the  moon's  orbit  being  very  oblique  to  the  horizon.  At  these  times  it  is 
called  harvest-moon. 

The  moon  has  very  little,  if  any,  atmosphere.  It  appears  covered 
with  dark  and  light  spots,  caused  by  mountains,  plains,  and  valleys.  It 
seems  to  have  no  large  bodies  of  water. 

On  the  moon  the  stars  would  appear  to  revolve  in  27^  days,  and  the 
sun  in  29^  days.  Though  the  moon  reflects  considerable  light  to  the 
earth,  it  has  been  demonstrated  that  she  reflects  no  heat.  The  eccen- 
tricity of  her  orbit  is  13,333  miles. 


ASTRONOMY. 


445 


CHAPTER    XVIII. 

(CHART  NO.  10.) 

ZODIAC,     SEASONS,     TRANSITS,     PARALLAX,     ETC. 

865.  Figure  1.— The  zodiac.— The  zodiac  is  an  imaginary  cir- 
cular belt  or  zone  in  the  heavens,  16  degrees  wide;  extending  8  degrees 
on  each  side  of  the  ecliptic,  within  the  limits  of  which  lie  the  orbits  of 

FIG.  1. 


all  the  planets,  except  Ceres,  Pallas,  and  Juno.  The  middle  circle,  in 
the  figure,  represents  the  ecliptic,  and  the  space  included  between  the 
other  two  circles  represents  the  zodiac.  The  name  zodiac,  signifying  an 


440  ASTRONOMY. 

animal,  is  given  to  this  belt  because  each  of  the  12  signs  formerly  rep- 
resented some  animal. 

866.  The  signs  or  constellations  of  the  zodiac  (Fig.  1).— 
The  zodiac  is  divided  into  12  equal  parts,  called  signs  or  constellations  ; 
shown  by  the  division  lines  crossing  the  circles.  Each  of  these  signs 
or  divisions  contains,  of  course,  30  degrees,  each  degree  60  minutes,  and 
each  minute  60  seconds. 

The  names,  order,  and  symbols  of  the  twelve  signs  of  the  zodiac,  are 
as  follows : 


Aries,  or  the  Ram  ........    T 

Taurus,  the  Bull  .........    « 

Gemini,  the  Twins  ........    n 

Cancer,  the  Crab 
Leo,  the  Lion 
Virgo,  the  Virgin 


Libra,  the  Balance ^=. 

Scorpio,  the  Scorpion IT 

Sagittarius,  the  Archer. ...    / 

Oapricornus,  the  Goat V5 

Aquarius,  the  Waterman . .   $y 
Pisces,  the  Fishes }£ 


Philosophy  of  the  Seasons. 

867.  Day  an(i  night  (Fig.  1). — Day  and  night  are  due  to  the 
rotation  of  the  earth  on  its  axis.     The  sun  is  continually  rising  to 
places  in  the  west,  and  continually  setting  to  places  in  the  east.     But 
days  and  nights  differ  in  length  in  different  places  at  the  same  time, 
and  in  the  same  places  at  different  times.     The  causes  of  these  varia- 
tions will  be  explained  presently. 

868.  Causes  of  the  seasons  (Fig.  1). — The  seasons,  and  unequal 
lengths  of  the  days  and  nights,  are  caused  by  the  earth's  revolution 
around  the  sun,  with  its  own  axis  inclined  to  the  plane  of  its  own  orbit. 

The  earth,  in  the  figure,  is  represented  in  its  several  positions,  as 
regards  the  zodiac  and  its  own  orbit,  in  which  it  will  be  found,  respec- 
tively, on  the  20th  of  each  month,  and  as  it  is  just  entering  each  sign 
of  the  zodiac ;  as  shown  by  the  names  of  the  months  and  constella- 
tions. Its  north  pole  is  turned  toward  the  observer,  and  its  axis  in- 
clined 23°  28'  to  the  plane  of  its  orbit,  which  is  represented  by  the 
surface  of  the  paper  (or  chart).  The  direction  of  its  motions,  on  its 
axis  and  around  the  sun,  is  indicated  by  the  arrows. 

The  centre  of  the  sun  is  to  the  left  of  the  vertical  dotted  line 
1,618,000  miles,  which  is  the  eccentricity  of  the  earth's  orbit. 

869.  The  earth  at  the  solstitial  points  (Fig.  1).— On  the 
extreme  right  the  earth  is  seen  in  the  position  of  its  north  pole,  N, 
inclined  directly  toward,  and  its  south  pole  directly  from,  the  sun.     In 
this  position,  its  northern  hemisphere  is  more  directly  exposed  to  the 


ASTRONOMY.  447 

sun,  and,  consequently,  is  favored  with  summer,  and  the  longest  days 
and  shortest  nights  of  the  year;  while  the  southern  hemisphere,  being 
partially  turned  from  the  sun,  is  subjected  to  winter,  and  the  longest 
nights  and  shortest  days.  Yet,  in  this  position,  the  earth  is  in  its 
aphelion,  and,  consequently,  at  its  greatest  distance  from  the  sun,  being 
3,236,000  miles  (or  twice  its  eccentricity)  further  from  the  sun  than 
•when  it  is  at  its  perihelion.  The  earth  is  in  this  position  on  the  21st 
of  June,  called  the  summer  solstice. 

In  the  progress  of  the  earth,  for  six  months,  from  this  point  around 
to  its  perihelion,  on  the  extreme  left,  the  days  of  the  northern  hemi- 
sphere grow  shorter  and  the  nights  longer;  while  in  the  southern  hem- 
isphere the  days  grow  longer  and  the  nights  shorter.  In  this  position, 
the  south  pole,  S,  being  inclined  directly  toward  and  the  north  pole 
directly  from  the  sun,  it  is  winter  in  the  northern  hemisphere,  and  the 
days  are  shortest  and  the  nights  longest  of  any  in  the  year;  while  in 
the  southern  hemisphere  it  is  summer,  and  the  days  are  longest  and 
the  nights  shortest.  The  earth  is  in  this  position  on  the  22d  of  Decem- 
ber, called  the  winter  solstice. 

As  the  earth  passes  on  in  its  orbit  from  the  winter  solstice  back  to 
the  summer  solstice  again,  the  same  changes  occur,  but  in  the  reversed 
order. 

When  the  sun  reaches  its  greatest  northern  or  southern  declination, 
it  seems  to  stand  for  several  days  without  any  change  in  declination ; 
hence,  these  are  called  solstitial  points  of  the  ecliptic ;  solstitial  signi- 
fying, the  sim  standing  still. 

870.  The  earth  at  the  equinoctial  points  (Fig.  1). — Dur- 
ing the  three  months,  from  June  21st  to  September  22d,  the  earth 
passes  from  the  summer  solstice  to  the  autumnal  equinox  ;  and  during 
the  three  months,  from  December  22d  to  March  21st,  the  earth  passes 
from  the  winter  solstice  to  the  vernal  equinox.     At  the  equinoctial 
points  the  sun  is  directly  over  the  equator,  and  consequently  the  days 
and  nights  are  of  equal  length,  in  both  the  northern  and  southern 
hemisphere ;  hence  the  term  equinox,  which   signifies  equal  days  and 
nights. 

871.  The  sun's  declination. — The  apparent  distance  of  the 
sun  north  or  south  of  the  equator,  is  called  its  declination.     When 
north  of  the  equator,  it  is  called  northern  declination  ;  when  south  of 
the  equator,  it  is  called  southern  declination.     The  amount  of  the  de- 
clination is  23£  degrees ;  that  is,  23J  degrees  north,  and  23£  degrees 
south.      This   subject  will   be   referred   to  when  describing   Fig.   22 
(931). 


448  ASTRONOMY. 

872.  Constellations  of  the  zodiac  (Fig.  1). — A  sign  is  merely 
the  twelfth  part  of  a  circle.     Along  the  zodiac,  and  on  both  sides  of  it, 
are  seen  many  stars.     The  ancients  imagined  that  some  of  these  stars 
along  in   the  zodiac,  taken  together,  resembled  certain  objects,  and, 
consequently,  gave  such  clusters  or  groupings  of  them  corresponding 
names,  as,  the  Goat,  the  Lion,  the  Bull,  etc.     Thus,  each  sign  of  the 
zodiac  came  to  be  designated  by  a  particular  constellation.     The  names 
of  the  signs,  by  these  constellations,  have  already  been  given  (866). 

873.  The  sun's  apparent  motion  in  the  ecliptic  (Fig.  1).— 
When  the  earth  is  in  the  sign  Libra,  the  sun  will  appear  to  be  in  the 
sign  Aries ;  and  as  the  earth  moves  on  to  Scorpio,  the  sun  will  appear 
to  move  to  Taurus ;  and  so  on.     Hence,  as  the  earth  moves  around  in 
its  orbit  every  year,  from  west  to  east,  the  sun  will  appear  to  pass,  in 
the  same  direction  and  time,  through  all  the  signs  of  the  zodiac.     Or, 
all  the  constellations  of  the  zodiac  seem  to  pass  by  the  sun  westward 
once  a  year. 

874-  Division  of  the  signs.— The  signs  are  divided  into  four 
divisions,  corresponding  to  the  seasons.  The  Spring  signs  are  Aries, 
Taurus,  Gemini;  the  Summer  signs  are  Cancer,  Leo,  Virgo;  the  Au- 
tumnal signs  are  Libra,  Scorpio,  Sagittarius;  the  Winter  signs  are 
Capricornus,  Aquarius,  Pisces.  While  the  sun  passes  through  these 
signs,  of  course,  the  earth  passes  through  the  corresponding  opposite 
signs. 

875.  The  recession  of  the  equinoxes  or  precession  of 
the  constellations  (Fig.  1). — The  plane  of  the  equinoctial  passes 
through  the  earth's  equator,  or,  rather,  the  equinoctial  is  the  equator 
of  the  earth,  passing  off  into  the  heavens  in  every  direction.  There- 
fore, the  equinoxes  are  the  two  opposite  points  in  the  earth's  orbit  where 
the  plane  of  the  ecliptic  intersects  the  plane  of  the  equinoctial.  Now 
this  intersection  of  these  two  planes  does  not  always  take  place  in  the 
same  point.  That  is,  the  plane  of  the  equinoctial  is  revolving  back- 
ward, at  a  rate  that  would  complete  an  entire  rotation  in  from  25,000 
to  26,000  years.  Consequently,  the  two  points  where  these  two  planes 
intersect  on  the  earth's  orbit  (which  are  the  points  of  the  two  equi- 
noxes, 877)  are  moving  backward  at  the  same  rate.  Therefore,  the 
equinoxes,  each  year,  will  fall  a  little  behind.  This  annual  falling  back 
of  the  equinoctial  points  is  called  the  precession  of  the  equinoxes  ;  but 
it  would  be  better  to  say  recession  of  the  equinoxes  and  precession  of 
the  constellations.  The  equinoxes  thus  recede  to  the  west  upon  the 
ecliptic  at  the  rate  of  50 J  seconds  of  a  degree  every  year. 


ASTRONOMY.  449 

Hence,  the  months  and  signs,  in  the  diagram,  do  not  agree,  the  earth 
entering  each  sign  about  the  31st  of  each  month.  This  subject  will  be 
referred  to  again  when  describing  Fig.  23  (939). 

876.  Longitude  in  the  heavens  (Fig.  1)  is  reckoned  on  the 
ecliptic  eastward  from  the  vernal  equinox,  or  beginning  with  the  first 
degree  of  the  sign  Aries.     When  the  sun  enters  Aries,  its  longitude  is 
nothing,  and  that  of  the  earth  is  180° ;  or  when  the  earth  enters  Aries, 
its  longitude  is  nothing,  and  that  of  the  sun  is  180°.     When  the  earth 
enters  Cancer,  its  longitude  is  90°,  and  that  of  the  sun,  270°,  and  so 
on  ;  as  will  appear  by  observing  the  diagram. 

877.  Figure  2.— Intersection  of  the  ecliptic  and  equi- 
noctial.—The  intersection  of  the  plane  of  the  ecliptic  with  the  plane 
of  the  equinoctial  forms  an  angle  of  23°  28',  called  the  obliquity  of  the 
ecliptic.     S  represents  the  earth  at  its  summer  solstice ;  N,  at  its  winter 

FIG.  2. 


solstice ;  E,  at  its  vernal  equinox ;  and  F,  at  its  autumnal  equinox. 
The  dotted  ellipse  lies  in  the  plane  of  the  equinoctial ;  and  the  other 
ellipse  in  the  plane  of  the  ecliptic.  The  equinoctial  points  are  where 
these  two  planes  intersect  on  the  earth's  orbit. 

It  will  be  noticed,  that,  at  the  summer  solstice,  S,  the  sun  shines 
upon  the  north  pole  of  the  earth;  and  at  the  winter  solstice,  upon  the 
south  pole,  as  indicated  by  the  arrows.  It  will  be  observed,  also,  that 
the  central  arrow  points  to  the  earth  over  the  tropic  of  Cancer,  at  the 
summer  solstice,  and  over  the  tropic  of  Capricorn  at  the  winter  sol- 
stice, both  of  which  are  23°  28'  from  the  equator  of  the  earth  ;  hence 
the  torrid  zone  of  the  earth  is  46°  56'  in  width. 

878.  Polar   inclination   and   seasons  of  the   different 
planets. — From  what  has  been  said  respecting  the  seasons,  it  will  be 

29 


450 


ASTRONOMY. 


seen  that  any  planet  whose  axis  is  not  perpendicular  to  the  plane  of  its 
orbit,  is  subject  to  all  the  variations  of  seasons,  difference  in  length  of 
days  and  nights,  etc.,  same  as  the  earth.  The  extent  of  these  variations, 
however,  depends  upon  the  amount  of  the  polar  inclination.  If  the 
axis  be  much  inclined,  as  in  the  case  of  Venus,  the  variations  will  be 
correspondingly  great ;  but  if  the  axis  is  only  slightly  inclined,  as  in 
the  case  of  Jupiter,  the  variations  will  be  correspondingly  limited ;  and 
if  the  axis  were  perpendicular  to  the  plane  of  the  orbit,  the  length  of 
days  and  nights  would  be  equal  in  all  parts  of  the  planet,  and  the 
climate  at  any  given  latitude  would  always  be  the  same. 

The  following  table  exhibits  the  polar  inclination,  greatest  declina- 
tion, and  ividtli  of  torrid  zone  of  different  planets : 


INC.    OF  AXIS. 

DECLINATION. 

TORRID   ZONE. 

Venus  

75°  00' 

75°  00' 

150°    00' 

Earth  

23    28 

23    28 

46    56 

Mars  

28    40 

28    40 

57    20 

Jupiter  

3      5 

3      5 

6     10 

Saturn  

30    00 

30    00 

60     00 

The  Sun  .  . 

7    20 

It  will  be  observed  that  the  inclination  equals  the  declination,  and 
that  the  torrid  zone  is  double  the  declination. 

Philosophy  of  Transits. 

879.  Transits. — Nodes  are  two  points  where  the  orbit  of  the 
moon  or  of  a  planet  intersects  the  plane  of  the  ecliptic. 

The  passage  of  Mercury  or  Venus  directly  between  the  earth  and  the 
sun,  and  apparently  over  the  disk  of  the  sun,  is  called  a  transit.  A 
transit,  therefore,  can  never  occur  except  when  the  interior  planet  is  in 
or  very  near  the  ecliptic.  The  earth  and  planet  must  be  on  the  same 
side  of  the  ecliptic  ;  the  planet  being  at  one  of  its  nodes,  and  the  earth 
on  the  line  of  its  nodes.  Mercury  and  Venus,  being  the  only  interior 
planets,  are  the  only  ones  that  can  make  transits  visible  to  us  ;  but  the 
earth  may  make  transits  visible  from  Mars,  the  Asteroids,  Jupiter,  and 
so  on. 


880.  Figure  3.— Transits  of  Mercury.— The  figure  repre- 
sents the  ecliptic  and  zodiac,  with  the  orbit  of  the  interior  planet, 
Mercury.  The  line  of  his  nodes,  ST,  as  shown,  is  in  the  16th  degree 
of  Taurus,  and  16th  degree  of  Scorpion.  Now,  if  the  earth  is  in  Taurus, 
on  the  line,  TS,  when  Mercury  is  at  his  ascending  node,  T,  he  will  seem 


ASTRONOMY. 


451 


to  pass  upward  over  the  sun's  disk,  like  a  dark  spot,  as  represented  in 
the  figure  in  the  line  of  the  arrow  drawn  through  the  sun. 

If  Mercury  is  at  his  descending  node,  S,  when  the  earth  is  in  the  16th 
degree  of  Scorpion,  he  will  seem  to  pass  downward  across  the  face  of 


FIG.  3. 


the  sun.  As  shown  in  the  diagram,  the  earth  passes  the  ascending 
node  of  Mercury  in  November,  and  the  descending  node  in  May.  The 
last  transit  of  Mercury  took  place  Nov.  4,  1868.  There  will  be  four 
more  during  the  present  century ;  two  in  May,  and  two  in  November. 


881.  The  calculation  of  transits.— To  calculate  transits,  at 
any  one  node,  it  is  only  necessary  to  find  what  number  of  revolutions 
of  the  interior  planet  are  equal  to  one,  or  any  number  of  revolutions  of 
the  earth  ;  or  when  the  earth  and  the  planet  will  again  meet  on  the 
line  of  the  planet's  nodes. 

In  the  case  of  Mercury,  this  ratio  is  as  87,969  is  to  365,256;  from 
which  it  is  found  that, 

7  revolutions  of  the  Earth  are  equal  to  29  of  Mercury. 
13  "  "  "        54  " 

33          "  "  "       137  " 

46  "  "  "       191  " 

Hence,  transits  of  Mercury,  at  the  same  node,  may  happen  at  intervals 
of  7,  13,  33,  46  years,  and  so  on. 

Upon  these  principles  all  transits  and  eclipses  are  calculated. 
The  following  is  a  list  of  the  Transits  of  Mercury  which  have  oc- 
curred during  the  present  century. 


Nov.  8,  1802. 
Nov.  11,  1815. 
Nov.  .4,  1822. 


May  5,  1832. 
Nov.  7,  1835. 
May  8,  1845. 


Nov.  9,  1848. 
Nov.  11,  1861. 
Nov.  4,  1868. 


452 


ASTRONOMY. 


2.  Figure  4. — Mercury's  oscillation. — Let  the  straight 
line,  joining  the  earth  and  the  sun,  represent  the  plane  of  the  ecliptic. 
Now,  when  an  interior  planet  is  in  this  plane,  as  represented  at  N,  it 


FIG.  4. 


may  appear  to  be  on  the  sun's  disk,  shown  by  the  dark  spot  on  the  sun  ; 
but  if  it  is  either  above  or  below  the  ecliptic,  as  shown  at  R  and  S,  it 
will  appear  to  pass  either  above  or  below  the  sun,  as  represented  at  A 
and  B.  If  Mercury  were  at  R,  it  would  appear  to  be  at  A,  and  in  44 
days  (that  is,  half  the  time  of  its  periodic  revolution)  it  would  be  as 
much  below  the  sun  as,  at  A,  it  appears  to  be  above  it;  and,  therefore, 
it  would  appear  to  be  at  B ;  and  so  on.  This  apparent  motion  of  an 
interior  planet,  from  west  to  east  and  from  east  to  west,  is  called 
oscillation. 

These  oscillations  do  not  take  place  in  half  the  time  of  the  planet's 
periodic  revolution,  because  the  earth,  in  the  meantime,  follows  the 
sun  in  the  same  direction.  Hence,  instead  of  occurring  (in  the  case  of 
Mercury)  in  44  days,  the  time  will  be  prolonged  to  between  55  and  65 

days. 

FIG.  5. 


883 .  Figure  5.  Inclination  of  the  moon's  orbit  to  the 
plane  of  the  ecliptic. — The  plane  of  the  moon's  orbit  is  very  near 
that  of  the  ecliptic.  It  departs  from  the  latter  only  5°  S'  48". 

Let  the  dotted  line  represent  the  plane  of  the  earth's  orbit  (which, 
of  course,  coincides  with  the  plane  of  the  ecliptic),  and  the  line  join- 
ing the  moon  at  M  and  N  would  represent  the  inclination  of  the  moon's 
orbit  to  that  of  the  earth.  At  N  the  moon  would  be  within  the  earth's 
orbit,  and  at  M  exterior ;  and  it  would  be  full  moon  at  M,  and  new 
moon  at  N. 


ASTRONOMY. 


453 


884-  View  of  the  moon  at  the  poles  and  at  the  equator 
of  the  earth. — As  the  full  moon  always  happens  when  the  moon  is 
directly  opposite  to  the  sun,  all  the  full  moons  in  our  winter  must  hap- 
pen when  the  moon  is  on  the  north  side  of  the  equinoctial ;  because 
then  the  sun  is  on  the  south  of  it.  Consequently,  at  the  north  pole  of 
the  earth,  there  will  be  alternately  a  fortnight's  moonlight  and  a  fort- 
night's darkness,  for  a  period  of  six  months;  and  the  same  will 
be  true  at  the  south  pole,  during  the  six  FIG.  6. 

months  that  the  sun  is  north  of  the  equi- 
noctial. 

About  the  equator  of  the  earth,  the  moon 
rises  throughout  the  year  with  nearly  equal 
intervals  of  delay,  from  one  day  to  another, 
of  48  minutes  and  44  seconds.  But  in  places 
of  considerable  latitude  a  deviation  from 
this  rule  occurs,  especially  about  the  time 
of  harvest,  when  the  full  moon  rises,  for 
several  nights  together,  only  18  to  25  min- 
utes later  each  day,  when  it  is  called  har- 
vest-moon ;  as  it  affords  the  farmer  extra 
light  for  gathering  crops. 

Parallax  of  the  Heavenly  Bodies. 

885.  Figure  6. — Annual  parallax, 
or  parallax  of  the  stars. — The  change 
in  the  apparent  position  of  the  fixed  stars, 
caused  by  the  change  of  the  earth's  place 
in  her  revolution  around  the  sun,  is  called 
the  annual  parallax. 

Let  L  represent  the  place  of  the  earth 
on  the  1st  of  January,  and  A,  a  star  ob- 
served at  that  time.  Its  apparent  place,  in 
the  more  distant  heavens,  will  be  at  H.  In 
six  months  the  earth  will  have  revolved 
around  to  the  position  of  N,  and  the  star, 
A,  will  appear  to  be  at  F.  The  angle  LAN 
will  constitute  the  angle  of  parallax. 

Although  the  distance  between  L  and  N 
is  190,000,000  miles,  yet  the  parallax  of  the 
star,  A,  is  less  than  -^  of  one  degree,  and 
the  lines  LA  and  NA,  therefore,  are  almost 
parallel.  Hence,  if  the  earth's  orbit  were 


±54  ASTRONOMY. 

filled  with  a  globe  of  fire,  190,000,000  miles  in  diameter,  and  viewed  from 
the  fixed  star,  A,  it  would  appear  but  a  point  of  light  one  minute  (!')  in 
diameter.  Therefore,  how  distant  and  immense  must  be  the  stars. 

Another  evidence  of  the  immense  distance  and  magnitude  of  the  stars 
is,  that  they  appear  no  nearer,  brighter,  or  larger,  when  viewed  from 
the  earth  at  S,  than  when  viewed  from  the  earth  at  R ;  although  at  S  the 
observer  is  190,000,000  miles  nearer  a  given  star  than  when  he  is  at  R. 

886.  Figure  7. — Diurnal  parallax. — This  applies  to  the  planets 
and  other  solar  bodies.     It  is  the  difference  between  the  altitude  of  a  solar 
body  seen  from  the  earth's  surface  and  the  altitude  of  the  same  body  seen 
at  the  same  time  from  the  earth's  centre.     Or  it  is  the  difference  between 
the  true  and  apparent  place  of  a  solar  body.     The  apparent  place  is 
that  in  which  the  body  seems  to  be  when  viewed  from  the  surface  of 
the  earth,  the  true  place  being  that  in  which  it  would  appear  if  seen 

FIG.  7.  from  the  centre  of  the  earth. 

If  an  observer  were  stationed  at  the 
centre  of  the  earth,  and  could  see  the 
moon  at  N,  it  would  seem  to  be  situat- 
ed among  the  stars  at  F ;  whereas,  if 
it  were  seen  from  the  surface  of  the 
earth  at  S,  it  would  appear  among  the 
stars  at  R.  Therefore,  F  is  the  true 
and  R  the  apparent  place  of  the  moon ; 
the  space  between  F  and  R  being  the 
arc  which  measures  the  moon's  paral- 
lax. 

The  greater  the  distance  the  less  the  parallax.  If  the  moon,  N,  were 
removed  to  T,  the  arc  which  would  then  measure  its  parallax  would 
be  included  between  J  and  R. 

The  horizontal  parallax  is  the  greatest.  That  is,  the  parallax  is 
greatest  when  the  body  is  on  the  sensible  horizon;  from  which  point 
it  diminishes,  until  it  reaches  the  zenith,  Z,  where  its  parallax  ceases, 
or  becomes  nothing.  Thus  it  will  be  seen  that  the  arc  NH  is  less  than 
that  of  FR;  hence,  the  parallax  of  the  moon  is  less  at  L  than  it  was 
atK. 

Diurnal  parallax  applies  only  to  bodies  of  the  solar  system.  The  stars 
are  too  far  off,  of  course,  to  show  any  difference  in  position  when  viewed 
from  points  so  near  together  as  the  centre  and  surface  of  the  earth. 

887.  The  effect  of  parallax  on  bodies  is  to  depress  them 
below  their  true  place.     On  this  account,  the  parallax  of  the  sun  and 
moon  must  be  added  to  their  apparent   altitude,  in  order  to  obtain 
their  true  altitude. 


ASTRONOMY. 


455 


888.  The   principles  of  parallax  are   of  great  impor- 
tance, as  by  them  the  distance  of  the  solar  bodies  from  the  earth,  the 
magnitude  of  the  planets,  and  the  dimensions  of  their  orbits,  may  all 
be  determined.     Having  thus  found  the  distance  of  the  earth  from  the 
sun,  that  of  all  the  planets  may  be  known  also ;  because,  according  to 
the  third  law  of  Kepler,  the  squares  of  the  times  of  their  sidereal 
periodic  revolutions  are  proportional  to  the  cubes  of  their  mean  dis- 
tances. 

889.  Figure  8.— Convexity  of  the  earth's  surface.— This 

is  shown,  1st.  By  the  manner  in  which  a  ship  disappears  from  sight,  as 
she  sails  in  any  direction  from  the  coast:  The  hull  or  body  of  the 
vessel  first  disappears,  then  the  rigging,  and  lastly  the  tops  of  the  masts 
vanish  from  sight. 

2d.  Navigators  have  sailed  around  the  earth,  and  thus  proved  its 
convexity. 

3d.  The  form  of  the  earth's  shadow,  as  seen  upon  the  moon  in  an 
eclipse,  proves  the  globular  figure  of  the  earth,  and  so  the  convexity 
of  its  surface. 

4th.  Latitude  found  by  the  north  star.  The  convexity  of  the  earth, 
north  and  south,  is  proved  by  the  variation  in  the  altitude  of  the  north 
star,  which  is  found  to  uniformly  increase  as  we  approach  it,  and  to 
diminish  as  we  recede  from  it. 

Suppose  an  observer  were  stand- 
ing upon  the  earth  (Fig.  8),  and 
viewing  the  pole-star  from  the 
45th  degree  of  north  latitude;  it 
would,  of  course,  appear  elevated 
45°  above  his  visible  horizon,  rep- 
resented by  the  arrow  H.  But 
let  him  recede  southward,  over 


FIG.  8. 


one  degree  of  latitude,  and  the 
pole-star  will  settle  one  degree  to- 
ward the  horizon ;  or,  rather,  his 
northern  horizon  would  be  elevat- 
ed one  degree  toward  the  star ;  till 
at  length,  as  he  crossed  the  equa- 
tor, his  horizon,  shown  by  the 
arrow,  S,  would  rise  above  the  star,  when  it  would  become  wholly  in- 
visible. 

Hence  the  general  rule,  that  the  altitude  of  one  pole,  or  the  depres- 
sion of  the  other  at  anyplace  on  the  earth's  surface,  is  equal  to  the 
latitude  of  that  place. 


456  ASTRONOMY. 

890.  Figure  9. — Conjunction  and  opposition  of  planets. 

— Conjunctions  are  called  inferior  and  superior.     When  Mercury  or 

Venus  is  nearest  to  the  earth,  that 

_^ is,  between  the  earth  and  sun,  as  at 

D,  it  is  in  inferior  conjunction  ; 
and  when  furthest  from  the  earth, 
as  at  N,  it  is  in  superior  conjunc- 
tion, in  which  case  the  sun  is  be- 
tween the  earth  and  the  planet. 

The  exterior  planets,  namely, 
those  whose  orbits  are  exterior  to 
that  of  the  earth,  have  alternately 
a  superior  conjunction  and  an  op- 
position. An  exterior  planet  is  in 
superior  conjunction  when  the  sun 
is  between  the  planet  and  the 
earth  ;  and  it  is  said  to  be  in  oppo- 
sition when  the  earth  is  between  the  sun  and  the  planet. 

Hence,  when  a  planet  is  in  conjunction,  it  rises  and  sets  nearly  with 
the  sun;  but  in  opposition,  it  rises  nearly  when  the  sun  sets,  and  sets 
when  the  sun  rises. 

When  at  her  superior  conjunction,  Venus  is  154,000,000  miles  from 
the  earth,  but  when  at  her  inferior  conjunction  she  is  only  26,000,000 
miles  distant.  The  reason  of  this  is  apparent. 

Venus  presents  all  the  phases  of  the  moon  in  passing  around  the  sun. 

891.  Direct,  stationary,  and  retrograde  motion  of  the 
planets  (Fig.  9). — The  planets,  if  seen  from  the  sun,  would  appear  to 
pass  from  star  to  star,  through  the  constellations,  in  a  uniform  and 
regular  manner.     But  as  seen  from  the  earth,  they  apparently  move 
irregularly.     Sometimes  they  appear  to  go  forward  ;  at  other  times,  to 
remain  stationary,  and  then  to  recede. 

When  Venus,  for  example  (Fig.  9),  is  at  T,  her  motion  is  said  to  be 
direct,  passing  from  west  to  east  toward  L.  When  at  F,  she  would  be 
coming  directly  toward  the  earth  at  E ;  therefore,  while  moving  from 
F  to  H,  she  would  seem  to  be  stationary.  In  passing  from  H  to  R 
(travelling  faster  than  the  earth)  she  will  pass  the  earth,  aijji  so  seem 
to  move  from  east  to  west,  or  to  retrograde.  From  R  to  S  she  would 
be  moving  directly  away  from  the  earth,  when  she  would  again  seem 
to  be  stationary. 

Of  course,  the  earth  has  been  moving  along  in  her  orbit,  which  is 
not  represented  in  the  diagram,  but  the  principle  sought  to  be  illus- 
trated will  be  understood. 


ASTRONOMY. 


457 


892.  The  transit  of  Venus  an  important  event.— If  the 

orbit  of  Venus  lay  exactly  in  the  plane  of  the  earth's  orbit,  she  would 
cross  the  sun's  disk,  like  a  dark  spot,  at  every  inferior  conjunction. 
But  her  orbit  cutting  the  ecliptic  at  an  angle  of  3£°,  she  will  pass  the 
sun  a  little  above  or  below  it,  except  when  her  inferior  conjunction 
happens  in  or  near  one  of  her  nodes ;  in  which  case  she  will  make  a 
transit,. which  can  happen  only  twice  in  a  century. 

Progress  in  astronomical  science  since  Venus'  last  transit  (in  1822) 
will  render  her  next  transit,  in  1874,  the  means  of  demonstrating  many 
truths,  and,  therefore,  one  of  the  most  important  events  of  the  age. 

The  following  is  a  list  of  all  the  transits  of  Venus,  since  the  time  the 
tirst  was  observed,  to  the  year  2012 : 


December  4th,  1639. 
June  5th,  1761. 

June  3d,  1769. 


December  6th,  1822. 
December  8th,  1874. 
December  5th,  1882. 


June 
June 


7th,  2004. 
5th,  2012. 


The  Periodic  Revolution  of  the  Sun. 

893.  Figure  10.— The  orbit  of  the  sun.— The  sun  has  three 
motions.  1st.  It  revolves  around  its  own  axis  once  in  25  days  9  hours 
36  minutes  (814).  2d.  It  revolves  around  the  centre  of  gravity  of  the 

FIG.  10. 


solar  system  (845).     3d.  It  has  a  periodic  revolution,  from  west  to  east, 
in  a  vast  orbit  around  some  distant  and  unknown  centre. 

A  portion  of  the  sun's  orbit  is  represented  by  the  line,  TH.     The 
point  of  tendency  is  toward  the  constellation  Hercules.     The  plane  of 


458  ASTRONOMY. 

this  vast  orbit  is  supposed  to  have  an  inclination  of  about  84°  to  the 
ecliptic.  The  sun  is  represented  as  surrounded  by  his  offspring,  the 
planets,  and  their  offspring,  the  satellites,  and  those  wandering  mem- 
bers of  the  solar  family,  the  comets. 

The  sun,  therefore,  is  not  stationary,  but,  taking  with  him  his  re- 
tinue of  worlds,  he  is  making  a  grand  tour  through  the  boundless 
Universe  of  God,  at  the  rate  of  20,000  miles  per  hour,  and  will  return 
to  the  same  point  of  his  orbit  only  once  in  every  eighteen  millions  of 
years. 

Reader,  think,  for  a  moment,  of  the  journey  you  are  taking  through 
space.  We  are  whirling  at  the  rate  of  over  a  thousand  miles  per  hour 
around  the  earth's  axis ;  and  sixty-eight  thousand  miles  per  hour  around 
the  sun ;  and  twenty  thousand  miles  per  hour  around  the  sun's  central 
orb ;  and  at  what  rate  we  may  be  journeying  around  some  other  grand 
centre  is  only  known  by  the  Divine  Mind. 


CHAP  TEE    XIX. 

PHILOSOPHY     OF     ECLIPSES. 

894-  Shadows  of  solar  bodies.— The  sun  being  larger  than 
the  planets  and  satellites,  the  principal  shadow  of  all  solar  bodies  is 
shaped  like  a  cone.  The  length  of  the  shadow  of  a  planet  or  satellite 
will  depend  upon  the  size  of  the  body  and  its  distance  from  the  sun. 
In  Fig.  12  (901),  for  instance,  it  will  be  seen  that  the  earth  on  the  left 
casts  a  longer  shadow  than  the  moon,  while  both  the  earth  and  moon 
on  the  right,  being  at  a  greater  distance  from  the  sun,  throw  longer 
shadows  than  the  same  bodies  do  on  the  left. 

Eespecting  the  umbra,  penumbra,  and  shadows  generally,  see  522 
and  523. 

All  the  planets,  both  primaries  and  secondaries,  cast  shadows  in  the 
direction  opposite  to  the  sun  (see  page  412). 

895.  Interest  felt  in  eclipses.— No  phenomena  of  the  heavens 
have  engaged  the  attention  of  mankind  more  than  eclipses  of  the  sun 
and  moon.  In  the  early  ages  they  were  regarded  as  alarming  devia- 
tions from  the  laws  of  nature,  presaging  public  calamities  and  indicat- 
ing divine  displeasure.  Even  at  the  present  day,  to  those  who  are  un- 
acquainted with  astronomy,  nothing  appears  more  wonderful  than  the 
accuracy  with  which  they  can  be  predicted.  They  can  be  calculated 
with  great  precision  for  ages,  either  past  or  to  come. 


ASTRONOMY.  459 

896.  Position  of  the  sun,  earth,  and  moon,  when  eclipses 
occur. — Eclipses  of  the  sun  can  happen  only  at  new  moon,  and  those 
of  the  moon  only  at  full  moon  ;  for  the  moon  can  never  be  between  us 
and  the  sun,  to  eclipse  him,  except  at  the  time  of  her  change  or  new 
moon ;  and  she  can  never  pass  into  the  earth's  shadow,  to  be  eclipsed 
herself,  except  when  she  is  in  opposition  to  the  sun,  and  it  is  full 
moon. 

An  eclipse  of  the  sun*oi  solar  eclipse,  is  caused  by  the  moon  passing 
between  the  earth  and  the  sun,  and  casting  her  shadow  upon  the  earth. 

An  eclipse  of  the  moon,  or  lunar  eclipse,  is  caused  by  her  falling  into 
the  earth's  shadow. 

897.  Eclipses  are  either  total,  partial,  or  annular. — When 
the  disk  of  the  sun  or  moon  is  wholly  obscured,  the  eclipse  is  total; 
when  only  partly  obscured,  the  eclipse  is  partial;  when  the  central 
part  of  the  sun's  disk  is  obscured,  leaving  a  bright  ring  around  the 
shadow,  the  eclipse  is  annular  (ring-like). 

The  apparent  diameter  of  the  sun  or  moon's  disk  is  divided  into 
twelve  equal  parts,  called  digits. 

FIG.  11. 


898.  Figure  11.— The  direction  in  which  eclipses  come 

on. — The  dotted  arrow  represents  the  orbit  of  the  earth ;  the  other  ar- 
rows, the  rotation  of  the  earth  on  its  axis,  and  the  revolution  of  the 
moon  in  its  orbit.  The  sun  is  seen  on  the  left.  U  represents  the 
earth's  umbra,  and  PP,  its  penumbra.  The  moon's  umbra  and  penum- 
bra are  seen  on  the  left  of  the  earth.  The  upper  side  of  the  figure 
is  the  east  side,  and  the  lower  side  the  west  side. 

Eclipses  of  the  sun  always  come  on  from  the  west,  and  pass  over 
eastward.  On  the  left  of  the  earth  the  moon  is  seen  revolving  east- 
ward, throwing  her  shadow  upon  the  earth,  and  hiding  the  western 
limb  of  the  sun.  Hence,  the  shadow  of  the  moon  passes  over  the  sur- 
face of  the  earth  from  west  to  east. 

899.  Total  eclipse  of  the  moon,  and  partial  eclipse  of 

the  sun  (Fig.  11).— The  moon,  in  passing  through  the  umbra,  U,  of 


4(50  ASTRONOMY. 

the  earth,  is  totally  eclipsed.  The  sun  is  totally  eclipsed  to  places  with- 
in the  umbra  of  the  moon  (except  in  cases  of  an  annular  eclipse),  and 
partially  eclipsed  to  places  outside  of  the  umbra. 

Before  the  moon  enters  the  earth's  umbra,  U,  the  earth's  penumbra, 
PP,  begins  to  intercept  the  light  of  the  sun,  or  to  cast  a  faint  shadow 
upon  her.  This  shadow  grows  darker  and  darker,  till  the  moon  enters 
the  umbra,  or  perfect  shadow  of  the  earth. 

If  the  moon  passes  through  the  side  of  the  shadow,  instead  of  its 
centre,  the  eclipse  will  be  partial  instead  of  total. 

900.  Dimensions  of  the  earth   and    moon's  shadows. — 
As  before  stated,  the  length  of  the  shadow  of  a  solar  body  depends  on 
the  distance  of  the  body  from  the  sun.   The  diameter  of  a  given  shadow 
falling  upon  a  body,  will  depend  upon  the  distance  between  the  body 
casting  and  the  body  receiving  the  shadow. 

.The  average  length  of  the  earth's  umbra  is  about  860,000  miles ;  and 
its  breadth,  at  the  distance  of  the  moon,  is  about  6,500  miles,  or  three 
times  the  moon's  diameter.  The  earth  and  moon  revolving  in  ellip- 
tical orbits,  will,  of  course,  cause  the  above  estimates  to  vary.  The 
earth's  umbra  varies  in  length  from  842,217  to  871,262  miles,  and  its 
diameter,  where  the  moon  passes  it,  varies  from  5,235  to  6,365  miles. 

The  average  length  of  the  moon's  umbra  is  236,000  miles.  It  varies 
from  221,148  to  252,638  miles,  according  to  its  distance  from  the  sun. 
Its  greatest  diameter,  at  the  distance  of  the  earth,  is  175  miles  ;  but  the 
penumbra  may  cover  a  space  on  the  earth  of  nearly  5,000  miles  in 
diameter. 

When  the  sun  is  at  his  greatest  and  the  moon  at  her  least  distance 
from  the  earth,  as  at  A  (Fig.  12),  her  shadow  will  extend  19,000  miles 
beyond  the  surface  of  the  earth.  But  when  the  sun  is  at  his  least  and 
the  moon  at  her  greatest  distance  from  the  earth,  as  at  P  (Fig.  12),  her 
shadow  will  not  reach  the  earth  by  20,000  miles. 

It  is  owing  to  these  variations  that  some  central  eclipses  of  the  sun 
are  total,  while  others  are  partial  and  annular. 

901.  Figure  12. — Total  and  annular  eclipses  of  the  sun. 

— At  A,  the  earth  is  at  her  aphelion,  or  most  distant  point  from  the 
sun,  consequently  the  shadows  of  the  earth  and  moon  will  be  of  the 
greatest  possible  length.  At  the  same  time  the  moon,  on  the  left  of  A, 
is  in  perigee,  or  its  point  nearest  possible  to  the  earth.  If,  therefore, 
under  these  conditions,  the  moon  passes  centrally  over  the  sun's  disk, 
the  eclipse  will  be  total.  In  order  to  bring  the  shadow  of  the  moon  to 
a  point  at  the  surface  of  the  earth,  the  sun  would  require  to  fill  the 
space  between  the  points  of  the  two  long  dotted  arrows. 


ASTRONOMY.  4fJl 

At  P  the  conditions  are  reversed.  The  earth  is  at  her  perihelion,  or 
nearest  position  to  the  sun,  consequently  the  shadows  of  the  earth  and 
moon  will  be  of  the  shortest  possible  length.  At  the  same  time,  the 
moon,  on  the  right  of  P,  will  be  in  apogee,  or  its  point  farthest  pos- 
sible from  the  earth.  I£  under  these  conditions,  the  moon  passes 


FIG.  12. 


centrally  over  the  disk  of  the  sun,  her  shadow  will  not  be  sufficiently 
long,  by  20,000  miles,  to  reach  the  earth,  and  so  cover  his  whole  disk 
or  face,  but  will  leave  a  ring,  apparently  around  herself,  unobscured,  as 
shown  by  the  appearance  of  the  sun  in  the  figure. 

The  eccentricity  of  the  earth's  orbit,  in  the  diagram,  is  very  much 
exaggerated,  the  better  to  illustrate  the  principles  explained. 

902.  The  duration  of  eclipses. — 1.  The  greatest  possible  dura- 
tion of  the  annular  appearance  of  a  solar  eclipse  is  12  minutes  and 
24  seconds. 

2.  The  greatest  possible  time  during  which  the  sun  can  be  totally 
eclipsed  to  any  part  of  the  earth  is  7  minutes  and  58  seconds. 

3.  The  moon  may  continue  totally  eclipsed  for  1  hour  and  45  minutes. 

90S.  The  general  effects  of  a  total  eclipse  of  the  sun,  is 

ta  darken  the  heavens  at  an  unusual  hour,  and,  therefore,  to  impress 
the  mind  with  a  peculiar  gloom.  The  animal  tribes  also  seem  to  be 
agitated  by  the  untimely  darkness.  The  temperature  declines,  and  the 
planets  and  stars  become  visible. 

904.  The  number  of  eclipses  in  any  one  year  cannot  be 
less  than  two,  nor  more  than  seven.  In  the  former  case,  they  will  be 
both  of  the  sun  ;  and  in  the  latter,  there  will  be  five  of  the  sun,  and 
two  of  the  moon. 

Eclipses,  both  of  the  sun  and  moon,  recur  in  nearly  the  same  order, 
and  at  the  same  intervals,  at  the  expiration  of  a  cycle  of  223  lunations, 
or  18  years  of  365  days  and  15  hours.  At  the  expiration  of  this  time, 


462  ASTRONOMY. 

the  sun  and  moon's  nodes  will  sustain  the  same  relation  to  each  other 
as  at  the  beginning,  and  a  new  cycle  of  eclipses  begins.  This  cycle  is 
called  the  period  of  the  eclipses. 

905.  Figure  13.— Why  eclipses  are  not  more  frequent. 

— If  the  moon's  orbit  lay  in  the  plane  of  the  ecliptic,  instead  of  forming 
an  angle  of  5°  9'  with  it,  as  represented  by  Fig.  5  (883),  there  would  be 
two  central  eclipses  every  month ;  namely,  one  of  the  sun  and  one  of 
the  moon.  But,  owing  to  this  inclination  of  the  moon's  orbit  to  the 

FIG.  13. 


plane  of  the  ecliptic,  it  is  evident  that  she  may  be  either  above  or  below 
the  ecliptic  at  the  time  of  her  conjunction  with  the  sun,  as  shown  at 
E  and  H,  in  the  figure,  so  she  will  seem  to  pass  either  above  or  below 
him,  and  will  not  cause  a  solar  eclipse.  For  the  same  reason,  the  moon 
may  pass  either  above  or  below  the  earth's  shadow,  as  at  N  and  F,  at 
the  time  of  her  opposition,  and  no  lunar  eclipse  occur. 

It  is,  therefore,  only  when  the  moon  is  at  or  near  one  of  her  nodes 
that  either  a  solar  or  lunar  eclipse  can  occur.  Respecting  nodes,  see 
Fig.  3  (879  and  880). 

906.  Retrograde    motion  of  the    moon's   nodes. — The 

moon's  nodes  do  not  remain  in  the  same  position,  with  respect  to  the 
earth  and  sun,  but  have  a  retrograde  motion  of  about  19°  in  a  year ;  so 
that  she  comes  around  to  the  same  node  in  19  days  less  than  a  year,  or 
in  346  days,  causing  the  eclipse  to  occur  sooner  every  year  by  about 
19  days. 

In  just  half  of  346  days,  viz.,  173  days,  the  moon  passes  her  other 
node,  on  the  opposite  side  of  the  ecliptic.  It  follows,  therefore,  that  at 
whatever  time  an  eclipse  occurs  at  either  node,  there  will  occur  another 
at  the  opposite  node  in  173  days  thereafter. 

907 .  Figure  14. — The  solar  and  lunar  ecliptic  limits. — 

It  is  not  necessary  that  the  earth  and  moon  should  be  exactly  on  the 
line  of  the  moon's  nodes,  in  order  to  produce  an  eclipse.  If  she  is 
within  17°  of  her  node,  at  the  time  of  her  change  or  conjunction,  she 
will  eclipse  the  sun ;  and  if  within  12°  of  her  node  at  her  full,  she  will 


ASTRONOMY.  463 

strike  into  the  earth's  shadow,  and  be  more  or  less  eclipsed.     These 
distances  are  called,  respectively,  the  solar  and  lunar  ecliptic  limits. 

Let  the  light  globes  represent  the  sun  at  different  positions  on  the 
ecliptic,  and  the  dark  spheres,  the  moon ;  and  the  line  running  through 
them,  the  plane  of  her  orbit.  Let  the  point,  G,  represent  the  node  of 
the  moon's  orbit.  Now  if  the  change  occur  when  the  moon  is  at  A, 

FIG.  14. 


she  will  pass  Mow  the  sun ;  if  when  at  B,  she  will  just  touch  his  lower 
limb.  At  B,  then,  she  will  eclipse  him  a  little ;  and  so  on,  to  G,  at 
which  point  the  eclipse  would  be  central,  and  either  total  or  annular. 

If  the  moon  is  at  H,  I,  J,  K,  or  L,  when  the  change  occurs,  she  will 
eclipse  the  upper  or  northern  limb  of  the  sun ;  but  if  she  is  at  M,  she 
will  pass  above  the  limb,  and  not  eclipse  him  at  all.  The  points  B  and 
L  represent  the  solar  ecliptic  limits. 

The  mean  ecliptic  limit  for  the  sun  is  16£°  on  each  side  of  the  node. 
The  mean  ecliptic  limit  for  the  moon  is  10£°  on  each  side  of  the  node. 

908.  Why  there  are  more  solar  than  lunar  eclipses.— 

As  just  explained,  there  are  33°  about  each  node  of  the  moon,  making 
in  all  66°  out  of  360°,  in  which  eclipses  of  the  sun  may  occur;  and  21 
about  each  node,  making  in  all  42°  out  of  360°,  in  which  eclipses  of  the 
moon  may  happen.  The  proportion,  therefore,  of  the  solar  to  the  lunar 
eclipses  is  as  66  to  42,  or  as  11  to  7.  Yet,  in  a  given  time,  there  are 
more  visible  lunar  than  solar  eclipses,  which  is  owing  to  the  fact  that 
lunar  eclipses  are  visible  to  a  whole  hemisphere,  while  a  solar  eclipse  is 
visible  to  only  a  small  portion  of  it. 

909.  Eclipses  or  occultation  Of  the  stars. — The  occultation 
of  a  star  is  caused  by  the  moon  coming  between  us  and  the  star,  and 
so  concealing  it  from  our  view.     This  is  a  frequent  phenomenon,  and 
one  interesting  to  observe ;  especially  at  new  moon.     The  star  occulted 
may  be  traced  to  the  very  border  of  the  moon's  eastern  limb,  when, 
suddenly,  it  goes  out.     In  a  short  time  it  reappears. 

910.  Eclipses  of  Jupiter's  moons.— Jupiter's  satellites,  as  a 
general  rule,  are  totally  eclipsed  at  every  revolution.     The  average 
number  of  eclipses  of  his  moons,  altogether,  amount  to  about  forty 
per  month. 


464 


ASTRONOMY. 


911.  Eclipses  Of  Saturn's  moons.— The  satellites  of  Saturn 
are  seldom  eclipsed.  On  account  of  the  great  inclination  of  their  orbits 
to  the  ecliptic,  they  are  not  eclipsed  but  twice  in  thirty  years,  when  the 
rings  of  the  planet  are  edgewise  toward  the  sun.  See  Figs.  1  and  9 
(852). 


CHAPTER    XX. 

PHILOSOPHY     OP    THE    TIDES. 

912.  Motion  of  the  water  of  the  earth.  —  Owing  to  the 
perfect  mobility  of  water,  it  is  influenced  by  heavenly  bodies,  and  by 
the  motions  of  the  earth  itself,  in  a  manner  diiferent  from  that  in  which 
the  earth  is  influenced  as  a  whole. 

The  water  of  the  earth,  covering  more  than  two-thirds  of  its  surface, 
is  in  regular  and  ceaseless  motion ;  alternately  rising  and  falling  at 
regular  intervals  in  all  parts  of  the  globe. 

The  rising  of  the  water,  in  some  portions  of  the  earth,  and  its  falling, 
at  the  same  time,  in  other  portions,  are  called  tides. 

The  rising  of  the  waters  is  called  flood  tide  ;  and  their  falling,  ebb 
tide.  There  are  two  flood  and  two  ebb  tides  every  25  hours.  The 
highest  and  lowest  points  to  which  they  reach  are  called,  respectively, 
high  and  low  tides. 

913.  The  tides  are  not  uniform  at  any  given  place,  either  as 
to  time  or  amount.     They  occur  about  50  minutes  later  every  day,  and 
sometimes  rise  much  higher  and  sink  much  lower  than  the  average. 
The  extraordinary  high  and  low  tides  are  called,  respectively,  spring 
and  neap  tides  (923). 

914-  The  principal  cause  of  the  tides  is  the  attraction  of 
FIG.  15.  the  sun  and  moon  upon  the  water  of  the 

ocean. 

The  height  of  the  water  in  all  the  fig- 
ures representing  the  tides,  is  designedly 
exaggerated,  to  better  illustrate  the  prin- 
ciples to  be  explained. 

915.  Figure  15. — Influence  of 
the  earth  upon  its  waters.— If  the 

water  was  not  influenced  by  the  attraction 
of  the  sun  and  moon,  it  would,  under  the 


ASTRONOMY. 


4«5 


influence  of  the  earth's  own  attraction  and  centrifugal  force,  come  to  a 
proper  level,  and,  except  for  the  disturbing  influence  of  the  wind,  re- 
main from  age  to  age  in  a  state  of  equilibrium,  as  represented  by  the 
diagram. 

916.  Figure  16.— A  single  tide- wave.— It  would  seem,  at 
first  thought,  that  the  natural  effect  of  the  moon's  attraction  would  be 
to  produce  a  single  tide-wave,  on  the  side  of  the  earth  toward  the  moon, 
as  represented  in  the  figure ;  in  which  the  moon  is  seen  in  its  orbit  on 

FIG.  16. 


the  right.  The  three  arrows  represent  the  attraction  of  the  moon  upon 
the  water  of  the  earth.  If  the  water  were  thus  drawn  to  only  one  side 
of  the  earth,  there  could  be  but  one  flood  and  one  ebb  tide  in  24  hours ; 
whereas  there  are  two  of  each. 

91 7.  Figure  17.— The  two  tide- waves.  —  Instead  of  only 
one  tide-wave,  as  illustrated  in  the  last  figure,  there  are  two,  situated 
directly  opposite  to  each  other,  as  shown  in  this  figure.  The  causes  of 

FTG.  17. 


the  opposite  tide-wave  will  be  explained  hereafter, 
these  two  high  tides  there  are  two  low  tides. 

30 


Half-way  between 


466 


ASTRONOMY. 


These  four  tides,  viz.,  two  high  and  two  low,  traverse  the  ocean  from 
east  to  west  every  day,  which  accounts  for  a  flood  and  ebb  tide  every 
twelve  hours. 

On  the  right  is  represented  the  moon  in  her  orbit,  the  three  arrows 
representing  her  attraction. 

918.  Figure  18.— Lagging  of  the  tide-wave  behind  the 
moon. — As  the  moon,  which  is  the  principal  cause  of  the  tides,  is  re- 
volving eastward,  and  comes  to  the  meridian  later  and  later  each  day, 
therefore  the  tides  are  about  50  minutes  later  each  day.  This  makes 
the  interval  between  the  successive  high  tides  12  hours  and  25  minutes. 

FIG.  18. 


Besides  this  daily  delay  with  the  moon,  the  highest  point  of  the  tide- 
wave  lags  behind,  or  east  of  the  moon,  about  46°,  so  that  the  high  tide 
does  not  occur  till  about  three  hours  after  the  moon  has  crossed  the 
meridian.  This  is  because  the  waters  do  not  at  once  yield  to  the  im- 
pulse of  the  moon's  attraction,  but  continue  to  rise  after  she  has  passed 
over. 

In  the  figure,  the  moon  is  seen  in  her  orbit.  The  arrow,  pointing  to 
the  moon,  represents  her  attraction.  The  dotted  line  between  the 
earth  and  moon  stands  over  the  meridian. 


919.  Figure   19.— Influence   of  the   sun  upon  tides. — 

Thus  far,  the  attraction  of  the  moon  has  been  mentioned  as  the  prin- 
cipal cause  of  the  tides;  but  the  sun  has  the  same  effect  as  the  moon, 
only  in  a  less  degree.  The  relative  influence  of  the  moon  and  sun  is 
about  as  3  to  1. 

Let  the  arrow  M  represent  the  attraction  of  the  moon,  and  the  arrow 
S  that  of  the  sun.  Then  the  sun  partially  neutralizes  the  influence  of 
the  moon,  and  a  very  low  tide,  called  the  neap  tide,  is  the  result. 


ASTRONOMY. 


467 


When,  however,  the  sun  and  moon  are  either  in  conjunction  or  oppo- 

FIG.  19. 


sition,  as  at  A  or  P,  Fig.  21  (923),  their  forces,  being  united,  produce  an 
extraordinary  tide,  called  the  spring  tide. 

Causes  of  the  Opposite  Tide-wave. 

920.  Figure  20.— Causes  of  the  opposite  tide-wave.— 

The  principal  cause  of  the  tide-wave  on  the  side  of  the  earth  opposite 
the  moon,  is  the  difference  of  the  moon's  attraction  on  the  opposite  sides 
of  the  earth.  The  moon  is  represented  in  its  orbit  on  the  right. 

The  diameter  of  the  earth  being  equal  to  about  ^  of  the  moon's  dis- 
tance, and  the  power  of  attraction  being  inversely  as  the  square  of  the 

FIG.  20. 


distance,  the  water  on  the  side  opposite  to  the  moon  will  be  attracted 
with  a  force  about  T^  less  than  that  which  attracts  the  water  on  the 
side  toward  the  moon ;  while  the  rigid  part  of  the  earth  would  be  at- 
tracted with  a  force  commensurate  with  the  square  of  the  distance  from 
its  centre  of  gravity  to  the  moon.  The  effect  of  this  unequal  attraction 
upon  the  earth,  and  the  waters  upon  its  opposite  sides,  is  to  elongate 
the  general  form  of  the  water,  in  the  direction  of  the  moon,  and  thus 
produce  the  two  opposite  tide-waves. 


468  ASTRONOMY. 


.  The  secondary  cause   of  the  opposite  tide-  wave 

(Fig.  20),  is  the  revolution  of  the  earth  around  the  common  centre  of 
gravity  of  the  earth  and  moon. 

If  the  earth  and  moon  were  connected  by  a  rod,  as  represented  in 
the  figure,  there  would  be  some  point,  as  the  centre  of  the  dotted  cir- 
cle, where  they  would  balance  each  other,  which  would  be  their  com- 
mon centre  of  gravity.  This  point  is  about  6,000  miles  from  the 
earth's  centre.  Therefore,  every  time  the  moon  revolves  around  the 
earth,  or  rather  around  this  common  centre  of  gravity,  the  earth's  cen- 
tre will  revolve  around  the  same  point,  as  indicated  by  the  dotted  cir- 
cle, and  the  direction  of  the  arrow. 

By  this  revolution  of  the  earth,  the  water  on  the  side  opposite  the 
moon  is  subjected  to  greater  centrifugal  force  than  that  on  the  side  to- 
ward the  moon  ;  which  assists,  though  but  slightly,  in  the  production 
of  the  opposite  tide-wave. 

922.  Relative  influence  of  the  sun  and  moon  on  the  tides. 

—  The  tides  are  not  due  so  much  to  the  attraction  of  the  sun  and  moon, 
as  a  whole,  as  to  the  difference  of  their  attraction  on  the  opposite  sides 
of  the  earth.  The  attraction  being  inversely  as  the  square  of  the  dis- 
tance, the  influence  of  the  sun  and  moon,  respectively,  must  be  in  the 
ratio  of  the  earth's  diameter  to  their  distances. 

Now,  the  difference,  as  before  stated,  in  the  distance  of  the  two  oppo- 
site sides  of  the  earth  from  the  moon,  is  -fa  of  the  moon's  distance,  as 
240,000  ^-  8,000  —  30  ;  while  the  difference,  as  compared  with  the  dis- 
tance of  the  sun,  is  only  Trhr'  as  95,000,000  -f-  8,000  =  11,875. 

Hence,  although  the  sun,  as  a  whole,  attracts  the  earth  much  more 
than  the  moon  does  ;  yet,  because  of  her  greater  inequality  of  attrac- 
tion on  the  opposite  sides  of  the  earth,  the  moon  contributes  more  than 
the  sun  to  the  production  of  the  tides.  Their  relative  influence  is  as 
three  to  one.  When  acting  together,  they  produce  tides  one-third 
higher  than  usual;  when  counteracting  each  other,  the  lunar  tide-wave 
prevails,  but  is  one-third  lower  than  usual. 

Spring  and  Neap  Tides. 

923.  Figure  21.—  Spring  and  neap  tides.—  As  the  tides  are 
caused  by  the  attraction  of  both  the  sun  and  moon,  and  as  these  are 
constantly  changing  their  positions,  with  respect  to  the  earth   and  to 
each  other,  therefore  they  sometimes  act  one  against  the  other,  and 
partially  neutralize  each  other's  influence  ;  while,  at  other  times,  they 
combine  their  forces  and  mutually  assist  each  other  (919). 

The  dotted  ellipse  represents  the  earth's  orbit  ;  the  small  ellipses, 
the  moon's  orbit.  When  the  moon  is  in  quadrature,  as  seen  in  her  orbit 


ASTRONOMY. 


469 


at  N  and  T,  her  influence  is  measurably  neutralized  by  the  sun,  caus- 
ing low  tides,  called  neap  tides.  These  will  occur  at  both  quadratures 
of  the  moon.  When  the  moon  is  in  conjunction  or  opposition,  as  seen 
in  her  orbit  at  A  and  P,  high  tides  occur,  called  spring  tides.  These 
will  occur,  of  course,  at  both  full  and  new  moon. 
As  the  moon,  in  revolving  once  around  the  earth,  will  be  once  in 

FIG.  21. 


conjunction,  twice  in  quadrature,  and  once  in  opposition,  there  will  be 
two  neap  and  two  spring  tides  during  every  lunation.  Thus :  spring 
tide  at  conjunction,  neap  tide  at  first  quarter,  spring  tide  again  at  op- 
position, aud  neap  tide  again  at  second  quarter. 

92 Jf-  Variations  in  the  spring  tides  (Fig.  21). — The  distance 
between  the  earth  and  sun,  as  also  between  the  moon  and  sun,  is  differ- 
ent at  different  times  of  the  year ;  as  also  the  distance  between  the 
earth  and  moon  is  different  at  different  times  of  the  month.  Therefore, 
the  spring  and  neap  tides  are  not  always  alike,  as  to  their  elevation 
and  depression. 

At  A,  the  earth  is  in  aphelion,  its  greatest  possible  distance  from  the 
sun  ;  and  the  moon  is  in  apogee,  its  greatest  distance  from  the  earth. 
Therefore,  the  waters  of  the  earth  are  at  their  greatest  possible  distance 
from  both  the  sun  and  moon,  consequently  they  are  the  least  attracted 
by  them.  Hence,  the  spring  tides  are  correspondingly  low. 

At  P,  the  earth  is  in  perihelion,  its  least  possible  distance  from  the 
sun ;  and  the  moon  is  in  perigee,  its  least  distance  from  the  earth. 
Therefore  the  waters  of  the  earth  are  now  at  their  least  possible  dis- 
tance from  both  the  sun  and  moon,  consequently  they  are  subjected  to 
their  greatest  influence.  Hence,  the  spring  tides  are  highest. 


470  ASTRONOMY. 

925.  Tides  affected  by  declination.— The  tide- wave  tends  to 
rise  directly  under  the  sun  and  moon.     Hence,  at  the  time  of  the  equi- 
noxes, the  sun  being  over  the  equator,  and  the  moon  within  5£°  of  it, 
the  crest  of  the  great  tide- wave  will  be  on  the  equator,  as  represented 
in  Fig.  19  (919). 

As  the  sun  and  moon  decline  south,  one  tide-wave  forms  in  the  south, 
and  the  opposite  one  in  the  north.  Suppose  the  moon  and  sun  to  be  in 
the  south,  over  or  near  the  Tropic  of  Capricorn,  as  shown  in  Fig.  17 
(917) ;  then  the  highest  wave,  in  the  southern  hemisphere,  will  be 
about  3  o'clock,  P.M.,  and  the  lowest  about  3  o'clock,  A.M.  ;  while  at  the 
north,  over  or  near  the  Tropic  of  Cancer,  this  order  is  reversed.  If  a 
straight  line  be  drawn  from  the  crest  of  one  wave  to  that  of  the  other, 
it  will  be  seen  that  it  is  highest  tide  in  the  day-time  over  the  southern 
tropic,  and  highest  tide  in  the  night-time  over  the  northern  tropic. 

It  is  on  this  account  that,  in  high  latitudes,  every  alternate  tide  is 
higher  than  the  intermediate  ones ;  the  evening  tides  in  summer  (at 
the  north)  exceeding  the  morning  tides ;  and  the  morning  tides  in 
winter  exceeding  those  of  evening. 

Other  Causes  Affecting-  Tides. 

926.  The  winds  affect  the  time  and  character  of  tides. 

— Strong  winds,  according  to  their  direction,  may  either  retard  or 
hasten  the  rise  and  fall  of  tides,  or  may  increase  or  diminish  their 
height. 

927.  The  conformation  of  the  land  affects  the  time  and 
character  of  tides. — The  tide  will  be  later  or  longer  in  rising  in  a 
large  bay,  with  but  a  narrow  opening  into  the  sea  or  ocean.     Hence, 
in  the  large  Bay  of  New  York,  which  has  a  very  narrow  inlet,  it  is  not 
usually  high  tide  till  eight  or  nine  hours  after  the  moon  has  passed  the 
meridian. 

In  the  oceans,  especially  the  Pacific,  the  tide  rises  and  falls  but  a  few 
feet.  When  pressed  into  narrow  bays  or  channels  it  rises  higher. 

928.  The  average  elevation  of  tides,  at  a  few  points  on  our 
coast,  is  as  follows :  Cumberland,  at  the  head  of  the  Bay  of  Fundy,  71 
feet;  Boston,  11  £  feet ;  New  Haven,  8  feet ;  New  York,  5  feet ;  Charles- 
ton, N.  C.,  6  feet. 

929.  The  different  heights  of  water  in  different  oceans 
and  seas. — As  the  great  tide- waves  proceed  from  east  to  west,  they 
are  arrested  by  the  eastern  side  of  the  continents.     For  this  reason,  the 
water  is  20  feet  higher  in  the  Gulf  of  Mexico  than  in  the  Pacific  ocean, 


ASTRONOMY. 


471 


on  the  other  side  of  the  Isthmus.     The  Red  Sea  is  30  feet  higher  than 
the  Mediterranean.    Inland  seas  and  lakes  have  no  perceptible  tides. 

930.  Atmospherical  tides.— There  is  no  doubt  but  that  the 
same  influences  which  cause  tides  of  the  sea,  produce  tides  of  the  atmo- 
sphere. The  air  being  lighter  than  water,  and  nearer  to  the  moon,  the 
atmospherical  tides  must  be  correspondingly  higher  than  those  of  the 
sea.  According  to  Herschel,  these  tides  are,  by  very  delicate  observa- 
tions, rendered  not  only  sensible,  but  measurable. 

FIG.  23. 


The  Sun's  Declination.— Zones  and  Temperature. 

931.  Figure  22.— The  declination  of  the  sun.— This  fig- 
ure represents  the  direction  in  which  the  rays  of  the  sun  fall  upon  the 
earth,  when  the  latter  is  at  her  solstitial  and  equinoctial  points.^ 

The  student  should  compare  the  following  explanation  with  868, 
869,  870,  and  871. 


472  ASTRONOMY. 

The  dotted  semicircle,  N,  represents  the  position  of  the  sun  when 
the  earth  is  at  her  summer  solstice  ;  the  semicircle,  S,  his  position  when 
the  earth  is  at  her  winter  solstice  ;  the  semicircle,  E,  his  position  when 
the  earth  is  at  her  equinoctial  points. 

932.  The   zones.—  The  torrid  zone  (Fig.  22).—  The  angles, 
ERS  and  ERN,  are  nearly  23J  degrees  each  ;  hence  the  angle,  NRS,  is 
47  degrees  ;  and  the  surface  of  the  earth  included  between  the  lines, 
NR  and  SR,  constitutes  the  torrid  zone,  on  which  the  sun  shines  twice 
every  year  with  perpendicular  rays. 

The  frigid  zones.  —  When  the  sun  is  at  8,  it  shines  upon  the 
south  pole,  P,  and  the  extreme  rays  on  the  right  pass  by  the  pole  and 
are  tangent  to  the  earth  at  the  point,  H  23J  degrees  beyond  the  pole. 
When  the  sun  is  at  E,  he  shines  upon  both  poles  ;  and  when  at  N,  he 
shines  upon  the  north  pole,  O,  and  23  J  degrees  beyond  it;  while  the 
extreme  rays  on  the  right  are  tangent  to  the  earth  at  a  point  23^  de- 
grees above  the  south  pole,  P  ;  and  the  surface  of  the  earth  included 
between  this  point  and  the  point  H,  constitutes  the  south  frigid  zone, 
which  extends  23J  degrees  in  every  direction  from  the  pole. 

The  temperate  zones.  —  The  surface  of  the  earth  included  be- 
tween the  torrid  zone  and  south  frigid  zone  is  the  south  temperate  zone. 
The  northern  hemisphere  is,  of  course,  divided  in  the  same  manner. 

933.  When  the  sun  shines  on  the  poles  (Fig.  22).—  The 
sun  shines  on  the  south  pole  constantly  while  he  passes  from  E  to  S 
and  from  S  back  to  E  again,  which  will  occur  while  the  earth  passes 
from  the  autumnal  to  the  vernal  equinox,  including  the  six  months 
from  September  23d  to  March  21st.     During  this  time,  which  is  the 
period  of  the  sun's  southern  declination,  the  north  pole  will  be  in  dark- 
ness.    The  sun  shines  on  the  north  pole  constantly  while  he  passes  from 
E  to  N  and  from  N  back  to  E  again,  which  will  occur  while  the  earth 
passes  from  the  vernal  to  the  autumnal  equinox,  including  the  six 
months  from  March  21st  to  September  23d.     During  this  time,  which 
is  the  period  of  the  northern  declination,  the  south  pole  will  be  in 
darkness. 


934-  ^ke  effect  °f  the  sun's  declination  on  temperature 

is  due  to  the  manner  in  which  his  rays  strike  upon  the  surface  of  the 
earth.  Those  parts  of  the  earth  upon  which  the  rays  fall  perpendicu- 
larly are  always  warmest;  while  those  portions  upon  which  they  fall 
obliquely  are  comparatively  cold. 

In  the  diagram  (Fig.  22),  the  sun  is  supposed  to  be  at  S,  which 
causes  his  rays  to  fall  on  the  south  pole,  P,  and  the  shadow  of  the 


ASTRONOMY. 


473 


earth  to  fall  on  the  north  pole,  0.  A  ray  of  light,  therefore,  from  the 
centre  of  S  would  strike  at  NY  (New  York)  and  glance  off  in  the 
direction  of  T,  nearly  in  a  straight  line.  But  if  the  sun  were  at  E,  a 
ray  from  his  centre  would  fall  less  obliquely  at  New  York  and  be  re- 
flected in  the  direction  of  F.  If  the  sun  were  at  N,  a  ray  from  his 
centre  would  fall  upon  New  York  still  less  obliquely  and  be  reflected 
to  L,  nearly  in  a  perpendicular  direction  to  the  earth's  surface. 


CHAPTER    XXI. 

TERRESTRIAL    AND    CELESTIAL    GLOBES,    FIXED    8  TARS,   ETC. 

Latitude  and  Longitude. 

935.   Figure  23.— Celestial  and  terrestrial  latitude.— 

Celestial  latitude  is  reckoned  north  and  south  from  the  ecliptic,  B,  on 

FIG.  23. 


474  ASTRONOMY. 

a  circle  of  celestial  latitude,  and  not  from  the  equinoctial  or  celestial 
equator,  E.  Celestial  latitude,  therefore,  is  distance  north  or  south  of 
the  ecliptic ;  and  as  one  half  of  the  ecliptic,  B,  is  south  of  the  equinoc- 
tial (E)  and  the  earth's  equator,  it  follows  that  a  star  may  be  in  north 
celestial  latitude,  which  is,  nevertheless,  south  of  the  equinoctial. 

Terrestrial  latitude  is  distance  on  the  earth's  surface,  reckoned  in 
degrees  and  minutes,  north  or  south  from  the  equator  to  the  poles,  on 
any  meridian.  Hence  the  highest  latitude,  north  or  south,  is  90°. 

936.  Celestial  longitude  is  reckoned  from  the  vernal  equinox, 
or  the  first  degree  of  Aries  eastward,  around  the  ecliptic  to  the  same 
point  again;    therefore,  the  greatest  number  of  degrees  of  celestial 
longitude  is  360.     See  Fig.  1  (865). 

Terrestrial  longitude  is  distance  on  the  earth's  surface,  reckoned  east 
or  west,  from  any  given  meridian  on  the  equator  or  any  parallel  of  lati- 
tude. The  degrees  of  longitude  diminish  in  length  from  the  equator 
to  the  poles,  because  the  earth  grows  smaller  in  circumference.  The 
greatest  number  of  degrees  of  terrestrial  longitude  is  180  east  and  180 
west.  The  meridian,  from  which  the  reckoning  commences,  is  different 
in  different  countries.  The  English  reckon  from  that  passing  through 
Greenwich,  near  London ;  the  French  from  that  passing  through  Paris ; 
and  the  Americans  from  that  passing  through  Washington. 

937.  The  terrestrial  globe  represents  the  earth,  upon  which 
are  drawn  continents,  islands,  mountains,  oceans,  seas,  rivers,  parallels 
of  latitude  and  longitude,  boundaries  of  nations,  etc. 

938.  The  celestial  globe  (Fig.  23)  represents  the  heavens  as 
seen  from  the  earth,  upon  which  are  drawn  the  ninety-three  constella- 
tions, galaxy,  or  milky  way,  figures  of  various  animals  and  objects  from 
which  the  constellations  are  named,  and  circles  of  celestial  latitude 
and  longitude. 

There  are  ninety-three  constellations.  The  milky  way  is  composed 
of  a  vast  number  of  stars,  so  far  away,  and  situated  so  nearly  in  the 
same  direction,  as  to  appear  like  a  thin  cloud.  It  extends  from  north- 
east to  southwest  through  the  whole  circumference  of  the  heavens,  as 
represented  in  the  figure. 

TJie  celestial  poles  are  the  points,  N  and  S,  where  the  earth's  axis,  if 
extended,  would  meet  the  heavens. 

The  plane  of  a  meridian  extends  to  the  heavens,  and  forms  a  celestial 
meridian  or  circle  of  declination,  upon  which  are  measured  declination 
and  polar  distance;  the  declination  of  a  heavenly  body  being  its  dis- 
tance from  the  equinoctial  or  celestial  equator,  E,  north  or  south. 


ASTRONOMY.  475 

The  declination  and  polar  distance  always  equal  90°,  or  a  quarter  of 
a  circle. 

The  right  ascension  of  a  body  is  its  distance  east  of  the  first  point  of 
Aries,  measured  on  the  equinoctial,  whereas  celestial  longitude,  as  before 
stated,  is  reckoned  from  the  first  point  of  Aries,  measured  on  the  ecliptic. 

The  angle  of  right  ascension  is  included  between  the  meridian  pass- 
ing through  the  body,  and  the  one  passing  through  the  first  point  of 
Aries;  and,  like  celestial  longitude,  is  reckoned  360°. 

Circles  of  celestial  latitude  pass  through  the  poles  (A)  of  the  ecliptic, 
and  cut  its  plane  (B)  at  right  angles.  Upon  these  circles  the  latitude 
of  heavenly  bodies  is  measured,  north  and  south,  from  the  ecliptic. 

The  angle  of  longitude  is  included  between  the  circle  of  latitude 
passing  through  the  body,  and  the  one  passing  through  the  first  point 
of  Aries,  where  they  meet  at  the  poles  of  the  ecliptic. 

The  celestial  horizon  is  a  great  circle,  whose  plane,  passing  through 
the  centre  of  the  earth,  divides  the  heavens  into  two  hemispheres,  of 
which  the  upper  one  is  called  the  visible  hemisphere,  and  the  lower  one 
the  invisible  hemisphere.  It  is  the  plane  of  this  circle  which  deter- 
mines the  rising  and  setting  of  the  heavenly  bodies. 

The  sensible  horizon  is  the  circle  which  terminates  our  view,  where 
the  earth  and  sky  appear  to  meet. 

A  vertical  circle  is  a  great  circle  in  the  heavens,  passing  through  the 
zenith  and  nadir,  cutting  the  horizon  at  right  angles. 

The  meridian  is  that  vertical  circle  which  passes  through  the  north 
and  south  points  of  the  horizon. 

TJie  prime  vertical  circle  is  the  vertical  circle  which  passes  through 
the  east  and  west  points  of  the  horizon. 

Altitude  and  zenith  distance  are  measured  in  degrees  and  minutes 
on  vertical  circles. 

The  zenith  distance  of  a  heavenly  body  is  its  distance  from  the 
zenith.  The  altitude  and  zenith  distance  are  always  equal  to  90°. 

The  azimuth  of  a  heavenly  body  is  its  distance,  east  or  west,  from  the 
meridian. 

The  angle  of  azimuth  is  included  between  the  meridian  and  vertical 
circle  passing  through  the  body. 

The  amplitude  of  a  heavenly  body  is  its  distance,  north  or  south,  of 
the  prime  vertical  circle. 

Th,e  angle  of  amplitude  is  included  between  the  prime  vertical  and 
the  vertical  circle  passing  through  the  body. 

The  angles  expressing  azimuth  and  amplitude  are  formed  at  the  zenith 
where  the  vertical  circles  intersect  each  other ;  the  measurement  being 
made  on  the  rational  horizon. 

The  sums  of  the  azimuth  and  amplitude  are  always  equal  to  90°. 


476  ASTRONOMY. 

939.  Nutation   of  the   earth's   axis  (Fig.  23).— The   pre- 
cession of  the  equinoxes  (see  875)  consists  of  a  real  motion  of  the  pole 
of  the  heavens,  N,  among  the  stars,  in  a  small  circle,  NL,  around  the 
pole  of  the  ecliptic,  A,  as  a  centre,  keeping  constantly  at  its  present 
distance  of  23J  degrees  from  it,  in  a  direction  from  east  to  west,  and 
with  a  progress  so  slow  as  to  require  25,000  years  to  complete  one  revo- 
lution, called  the  Platonic  or  great  year.     Hence,  the  bright  star  of  the 
Lesser  Bear,  which  we  now  call  the  pole-star,  has  not  always  been,  nor 
will  always  continue  to  be,  the  polar  star ;  for,  in  12,500  years  from 
now,  the  north  pole  of  the  earth  will  be  at  L,  47  degrees  from  its  pres- 
ent position. 

This  revolution  will,  from  age  to  age,  cause  gradual  changes  in  the 
aspects  of  the  heavens. 

The  Fixed  Stars— Clusters— Nebulae— Galaxy,  etc. 

940.  Motion  of  the  stars. — The  stars,  instead  of  being  fixed  or 
stationary,  are  found,  like  our  sun,  to  be  revolving  around  their  own 
axes,  and  around  some  other  central  body,  and,  probably,  carrying  with 
them  systems  of  planets,  satellites,  and  comets. 

9 41-  Variable  or  periodical  stars  are  those  which  undergo  a 
regular  periodical  increase  and  diminution  of  lustre,  amounting,  in 
some  cases,  to  a  complete  extinction  and  revival. 

94%-  Temporary  stars,  or  new  and  lost  stars,  are  those 
which  have  appeared  from  time  to  time  in  different  parts  of  the  heavens, 
and,  after  remaining  for  a  while  apparently  immovable,  died  away  and 
left  no  trace  of  their  existence.  There  are  but  few  such  stars. 

943 .  Double  stars. — Many  stars,  which  to  the  naked  eye  appeal- 
single,  are  found,  by  aid  of  the  telescope,  to  consist  of  two  or  more  stars, 
situated  near  each  other.  These,  as  the  case  may  be,  are  called  dou- 
ble, triple,  or  multiple  stars ;  or  binary,  ternary,  etc. 

944-  Binary  systems. — When  two  or  more  stars  are  found  in  a 
state  of  revolution  about  their  common  centre  of  gravity,  as  the  planets 
revolve  around  our  sun  (845),  they  constitute  what  is  called  a  Unary 
or  stellar  system.  These  are  the  double  and  multiple  stars,  which  to 
the  naked  eye  appear  single.  There  have  been  discovered  about  6,000 
multiple  stars.  Their  periods  of  revolution  about  their  common  centre 
vary  from  a  few  to  thousands  of  years.  It  is  estimated,  in  some  cases 
which  have  been  observed,  that  the  smaller  revolve  about  the  larger  of 


ASTRONOMY.  47? 

the  component  stars  at  hundreds  of  billions  of  miles  from  each  other, 
with  the  astounding  velocity  of  millions  of  miles  per  hour. 

Besides  the  revolution  of  these  stars  (which  are  really  suns)  around 
each  other,  they  have  a  proper  motion  in  space,  like  our  sun,  around 
the  great  central  sun  of  our  cluster  (951). 

945.  Clusters  of  stars. — In  a  clear  moonless  night  there  can  be 
seen,  in  different  parts  of  the  heavens,  groups  of  stars  which  seem  to  be 
drawn  together,  as  if  by  some  special,  mutual  attraction.     The  Pleiades, 
or  the  Seven  Stars,  and  Hyades,  in  Taurus,  are  instances  of  this  kind. 
In  the  Pleiades  there  are  seen,  by  aid  of  the  telescope,  over  200  stars. 
Such  groups  are  called  clusters  of  stars.     Some  of  them  contain  ten 
or  twenty  thousand  stars,  apparently  compacted  and  held  together  in 
a  family,  apart  from  other  stars,  under  the  influence  of  other  laws  of 
aggregation  than  those  which  have  determined  the  scattering  of  stars 
over  the  general  apparent  surface  of  the  heavens.     Some  of  these  clus- 
ters are  of  a  globular  form,  while  others  have  a  very  irregular  figure. 

Nebulae. 

946.  Nebulae. — This  term  is  applied  to  those  clusters  of  stars  that 
are  so  distant  as  to  appear  only  like  a  faint  cloud  or  haze  of  light. 
There  is  probably  no  limit  to  the  number  of  nebulae.     Prof.  Mitchell 
concluded  that  some  of  these  are  so  distant,  that  their  light,  travelling 
192,000  miles  a  second,  would  not  reach  us  in  less  than  30,000,000  of 
years. 

Only  a  few  nebulse  are  seen  with  the  naked  eye.  When  seen  through 
the  telescope  they  appear  as  large  as  one-tenth  of  the  moon's  disk. 
Though  they  are  seen  in  all  parts  of  the  heavens,  they  are  most  nume- 
rous in  a  zone  of  the  heavens  at  right  angles  to  the  milky  way. 

947.  Classes  of  nebulae. — Nebulae  are  divided  into  five  classes, 
viz.: 

1st.  Resolved  nebulce,  or  those  which  have  been  discovered  to  be 
great  clusters  of  stars. 

2d.  Resolvable  nebula,  or  those  which  are  considered  to  be  composed 
of  stars. 

3d.  Stellar  nebulae,  which  have  an  oval  or  round  shape,  increasing  in 
density  toward  the  centre,  and,  sometimes,  seem  to  have  a  dim  star  in 
the  centre. 

4th.  Irresolvable  nebula,  which  are  considered  to  be  luminous  mat- 
ter in  an  atmospheric  state,  condensing  into  solid  bodies,  like  the  sun 
and  planets. 

5th.  Planetary  nebula,  which  resemble  the  disk  of  a  planet,  and  are 


478  ASTRONOMY. 

considered  to  be  in  an  uncondensed  state.  Some  of  these  are  of  enor- 
mous size.  There  is  one  situated  in  the  head  of  Aquarius,  computed 
to  be  of  sufficient  magnitude  to  fill  the  orbit  of  Uranus,  or  nearly 
11,000,000  of  miles  in  circumference.  A  few  nebulae  are  annular  in 
form.  There  are  also  double  nebula,  or  two  or  more  near  each  other. 

It  is  probable  that  all  nebulas  might  be  resolved  into  distinct  stars, 
had  we  telescopes  of  sufficient  power. 

948.  The  milky  way  an  annular  nebula. — The  vast  appa- 
rent extent  of  the  milky  way,  as  compared  with  other  nebulae,  is  owing 
to  its  comparative  nearness.     Could  we  view  the  galaxy  or  milky  way 
from  one  of  the  other  nebulae,  it  would  appear  as  an  ordinary  annular 
nebulae,  of  which  our  sun  is  one  of  the  stars.     Hence,  the  Milky  Way 
may  be  called  the  Great  Nebula  of  the  solar  system. 

949.  The  number  of  stars. — In  the  milky  way,  Herschel  esti- 
mated that  50,000  stars  passed  the  field  of  vision  of  his  telescope  in  a  sin- 
gle hour.     It  is  safe,  therefore,  to  estimate  the  number  of  stars  in  each 
cluster  or  nebula  as  almost  numberless.     Hence,  when  it  is  considered 
that  the  nebulae  themselves  are  also  numberless,  how  vast  must  be 
the  aggregate  number  of  stars  or  suns,  to  say  nothing  of  the  many 
times  as  many  planets,  satellites,  and  comets. 

950.  The  term  universe. — Each  nebula  and  cluster  is  called 
a  Universe  or  Firmament.     Therefore,  the  Universal  Whole  consists  of 
Firmament  upon  Firmament,  or  an  infinite  series  of  Universes. 

951.  Our   cluster,  or  firmament.— The  single  stars,  visible 
to  the  naked  eye,  including  the  milky  way  and  the  solar  system,  con- 
stitute only  one  of  the  clusters  or  nebulae  of  the  heavens ;  that  is,  only 
one  of  the  numberless  Universes  or   Firmaments,  which,  under  the 
guidance  of  the  Divine  Mind,  ceaselessly,  silently,  and  harmoniously 
traverse  the  boundless  realms  of  space.     Yet  all  is  made  up  of  atoms, 
inconceivably  small ;  and  the  wisdom,  power,  and  goodness  of  God  are 
not  less  manifest  in  the  infinitely  varied  combinations  of  the  infinites- 
imal molecules  of  matter  than  in  the  construction,  magnitude,  and 
splendor  of  the  heavens. 


INDEX. 


[THE  REFERENCES  ARE  TO  SECTIONS,  NOT  TO  PAGES.] 


ABERRATION,  chromatic.  437. 

of  lenses,  412. 

of  reflectors,  391. 
Absorptive  power,  causes  modifying,  297. 

for  heat,  295. 

for  light,  361. 

of  colors,  295. 

Accessory  properties  of  matter,  11-18. 
Accumulated  electricity,  690. 
Achromatic  combination  of  lenses,  438. 
Acoustics,  definition,  530. 
Action  and  reaction  are  equal,  65. 
Action  of  heat  on  matter,  202. 

of  surfaces  upon  heat,  290-297. 
Adaptation  of  the  eye  to  distance,  445. 
Adhesion,  37. 

distinguished  from  cohesion,  37. 
Affinity,  18. 
Air  an  aerial  ocean,  122. 

buoyancy  of,  140. 

condensed-air  fountain,  149. 

condenser,  148. 

gun,  150. 

impenetrability  of,  125. 

no  animal  life,  no  flight,  no  combustion, 
etc.,  without  it,  129. 

pump,  128. 

relation  of  to  earth,  same  as  glass  to  hot- 
house, 259. 

vibrating  in  tubes,  583.    (See  Atmosphere.) 
Alphabet,  telegraphic,  777. 
Amalgam,  677. 
Amalgamated  zinc,  734. 
Ampere's  electro-magnetic  theory,  766. 
Amplitude  of  a  heavenly  body,  938. 

angle  of,  938. 
Analogy  of  light  and  heat,  358. 

of  electricity  and  magnetism,  759. 
Analysis  of  colors  by  absorption,  422. 
Anemometers,  269. 
Animal  electricity,  789. 

heat,  cause  of,  213. 

respiration  dependent   upon    atmospheric 

pressure,  146. 
Animals,  electrical,  790. 
Annealing,  33. 
Aphelion,  a39. 
Application  of   voltaic  or  galvanic  electricity, 

745-758. 
Aqueducts,  98. 
Archimedes'  screw,  171. 
Armature  of  magnets,  601. 
Artesian  wells,  95. 
Artificial  magnets,  603. 

method  of  making,  604. 
Ascent  of  bodies,  55. 
Astatic  needle,  634,  764. 
Asteroids,  with  table,  824. 
Astronomical  phenomena,  early  observation  of, 

801. 
Astronomy,  definition,  792. 

descriptive,  physical,  and  practical,  793. 


Atmosphere,  122. 

action  of  suction  pumps  dependent  on,  132. 

action  of  the  syphon  dependent  on,  188. 

an  immense  heating  apparatus,  258. 

buoyancy  of,  shown  by  balloons,  140. 

composition  of,  124. 

compression  and  expansion  of,  127. 

density  of  at  different  altitudes,  139. 

effects  of  on  the  rising  and  setting  of  heav- 
enly bodies,  400. 

free  electricity  of,  710. 

height  of,  123. 

how  heated,  256. 

impenetrability  of,  125, 141. 

its  pressure  varies  with  altitude,  132. 

pressure  of  equal  in  all  directions,  130. 

pressure  of  shown  in  various  ways,  142-145. 

pressure  of  sustains  different  liquids  at  dif- 
ferent heights,  example  and  formula, 
13  >. 

pressure  of  varies  at  the  same  place,  133. 

pressure  of,  when  first  understood,  139. 

tides  of,  930. 

weight  or  pressure  of,  126. 
Atmospheric  electricity,  709-719. 

causes  of.  711. 

heat  of,  209. 
Atoms,  4. 
Attraction,  18. 

adhesive.  37. 


cohesive.  36. 

electrical,  656,  671. 

magnetic,  laws  of,  626. 

molecular,  18,  35. 

of  gravitation,  39. 

of  planets,  with  table,  822. 
Attraction  and  repulsion,  87. 
Audience  rooms,  553. 
Auroras  borealis,  719. 

australis,  719. 

effects  of,  719. 
Axes  of  lenses,  408. 
Axis,  secondary,  390. 

of  the  ecliptic,  847. 

of  the  equinoctial,  847. 

optical,  446. 
Azimuth,  938. 

angle  of,  938. 

B. 

BACKGROUND,  effects  of,  465. 

Balloons,  140. 

Barker's  mill,  163. 

Barometer,  its  construction  and  uses,  134. 

as  a  weather-glass,  136. 

height  of  the  mercury  at  different   alti- 
tudes, 135. 

wheel  form  of,  138. 

Barometric  changes  and  the  weather,  rule  for 
reading,  136. 

diurnal  variations,  137. 


480 


INDEX. 


Batteries,  735-740. 
Battery,  carbon,  739. 

discovery  of  voltaic,  724. 

effects  of  th«-voltaic,  745-758. 

galvanic,  725. 

Grove's  nitric  acid,  738. 

of  two  or  more  couples,  740. 

Smee's,  735. 

sulphate  of  copper,  736. 

Voltaic,  727. 
Beams  of  light,  362. 
Bellows,  hydrostatic,  111. 

pump,  178. 
Bells,  electrical,  686,  720. 

diving,  141. 

vibrations  of,  562. 
Belts,  source  of  electricity,  683. 
Binocular  vision,  469. 
Bodies,  centre  of  gravity  of,  40. 

laws  of  falling  and  rising,  55. 

luminous,       non-luminous,      transparent, 
translucent,  and  opaque,  359. 

magnetic,  and  magnetised,  615. 

method  of  electrifying,  661. 

relation  of  to  heat,  290-297. 

relation  of  to  light,  &59. 

stability  of,  43. 

suspended  without  contact,  774. 

visible,  emit  light  from  all  points,  363. 

weight  of,  39. 
Body,  definition  of,  1. 

electrified  by  induction,  668. 

heavenly,  792.     (See  Heavenly  Bodies.) 

solar,  807. 

Blasting  under  water  by  electricity,  706. 
Boats,  action  of  wind  on  sails  of,  59. 
Boiling  by  application  of  cold,  318. 
Boiling  point,  317. 

affected  by  altitude,  319. 

application  in  arts,  319. 

causes  modifying,  318,  320,  321. 

nature  of  vessel  varies,  321. 

solids  in  solution  varies,  320. 

table  of,   at    different    atmospheric  pres- 
sures, 334. 

Bohnenberger's  dry-pile  electroscope,  737. 
Boilers,  steam,  351. 

bursting  of,  230. 
Breast -wheel,  160. 

Breathing  dependent  upon   atmospheric    pres- 
sure, 146. 

Breezes,  land  and  sea,  264. 

Bridges,  suspension,  expansion  of  cables  of,  215. 
Brightness  of  the  ocular  image,  451. 
Brittleness,  32. 

Buckets  or  pots,  endless  chain  of,  167. 
Buildings,  how  heated.  255-257. 
Buoyancy  of  air,  140. 

of  liquids,  113. 
Burning-glasses,  301. 


C. 

CALCULATION  of  transits,  881. 
Caloric,  198. 
Calorimetry.  230. 
Camera  Lucida,  485. 

obscura,  439,  483,  484. 

similarity  between  and  the  eye.  440. 
Candle  bombs,  330. 
Capillarity,  18,  38. 
Carbon  battery.  739. 
Carbonic  acid,  density  of,  229. 
Catoptrics,  365. 
Caustic  curve,  391. 
Celestial  globe,  938. 

horizon,  938. 

longitude,  936. 

meridian,  938. 

poles,  938. 
Centre  of  gravity.  40. 

in  man,  46. 


Centre  of  gravity,  method  of  finding,  41. 

of  cubes,  43. 

of  pyramids,  44. 

of  solar  system,  845. 

of  vehicles,  45. 
Centrifugal  machine,  163. 

pumps,  169,  170. 

Centripetal  and  centrifugal  forces,  843. 
Chain -pump,  168. 

Changes  in  matter  chemical  or  physical,  2. 
Characteristic  properties  of  solids,  25-34. 
Charging  the  Leyden  jar,  693. 
Chemistry,  its  relation  to  physics,  8. 
Chemical  affinity.  2. 
Chord  in  music,  592. 
Chromatic  aberration,  437. 
Chromatics,  419. 

Circular  or  curvilinear  motion,  842. 
Circle,  vertical,  938. 

prime  vertical,  938. 
Climate,   influence  of  latent  heat  of  water  on, 

308. 
Clothing,  relations  to  heat,  252. 

relation  to  color,  295. 
Coercitive  force  of  magnets,  627. 
Co-existence  of  sound  waves,  568. 
Cohesion,  18,  36. 

among  solids.  36. 

in  solids,  liquids,  and  gases,  87. 
Cohesion  and  repulsion,  22. 

relation  of  in  the  three  states  of  matter,  23. 
Cold  and  heat  relative  terms.  199. 
Cold  by  evaporation,  335,  343. 
Color  blindness,  463. 
Color  of  the  electric  spark,  701. 
Colors,  analysis  of,  by  absorption,  422. 

complementary,  421. 

composition  of,  of  the  solar  spectrum,  424. 

dependent  on  amplitude  of  waves  of  light, 
50-2. 

different  effects  of,  on  vision,  464. 

of  opaque  bodies,  429. 

of  transparent  bodies,  430. 

primary,  419. 

union  of  two  primary  to  produce  a  second- 
ary, 423. 
Combination  of  waves  of  liquids,  565. 

of  translation  and  rotation,  19. 

of  the  five  mechanical  powers,  example 

and  formula*,  86. 
Combustion  a  source  of  heat,  211,  344. 

structure  of  flame,  344. 
Comets,  807. 

appearance  and  nature  of,  831. 

direction  of  the  motions  of,  835. 

orbits  of,  832. 

periodic  times  of,  833. 

the  number  of,  834. 
Comparison  of  thermometers,  273. 
Compass,  mariner's,  644. 

discovery  of,  644. 

tables  for  correcting  variations  of,  644, 
Compensating  pendulum,  60. 
Components  and  resultants,  58. 
Compound  levers,  69. 

wheel  and  axle,  74. 
Compressibility,  12. 

of  gases,  120. 

of  liquids,  89. 
Compression  a  source  of  heat,  236. 

of  the  earth  at  the  poles,  50,  850. 
Concave  lenses,  405. 

mirrors,  386. 

foci  of,  388,  389. 

images  by.  387. 
Condensation,  causes  of,  327. 

of  steam,  332. 
Condenser,  air,  148. 

electrical,  691. 

discharge  of  electrical.  694, 695. 
Conditions  affecting  terrestrial  gravity,  48,  51. 
Conductibility  of  clothing,  25->. 

of  crystals,  wood,  etc.,  244. 


INDEX. 


481 


Conductibility  of  gases,  249. 

of  liquids.  246^248. 

relative  of  moist  and  dry  air,  250. 

relative  of  solids,  liquids,  and  gases,  241. 

relative  of   solids  liquids,    and  gases  of 
the  same  temperature,  251. 

of  solids,  determination  of,  242. 

table  of,  of  solids,  242. 

varies  with  molecular  arrangement,  244. 
Conduction  of  electricity,  (558. 

of  heat,  240. 

heat  in  liquids  not  equalized  by,  247. 

musical  tones  caused  by,  243. 

the  principle  of  Davy's  safety  lamp,  245. 
Conductors  and  non-conductors  of  heat,  240. 

of  electricity,  658. 
Conjugate  foci,  properties  of,  389. 
Conjugate  mirrors,  reflection  of  heat  by,  292. 
Conjunction  and  opposition  of  planets,  890. 
Constellations  of  the  zodiac,  866,  872,  938. 
Construction  of  barometers,  134. 

of  thermometers,  271-283. 
Copper,  tempering,  33. 
Convection  of  heat,  253. 

in  liquids,  253. 

in  gases,  256. 

heating  buildings  by,  in  air,  256. 

heating  buildings  by,  in  fluids,  255. 

ocean  currents  caused  by,  254. 
Conversion  of  thermometric  scales,  273. 
Convexity  of  the  earth's  surface,  889. 
Convex  lenses,  408. 

conjugate  foci  of,  409. 

spherical  mirrors,  382. 

illustrated  by  plane  mirrors,  381. 

images  formed  by,  384,  385. 
Conveying  water  over  hills  with  syphons,  191. 
Cooling  by  radiation,  285. 
Copernicus'  theory  of  astronomy,  803. 
Cords,  vibration  of,  570-573. 
Coronas,  436. 

Couples,  simple  voltaic,  726. 
Crank,  irregular  action  of,  354. 
Cream,  why  rises  on  milk,  117. 
Cryophorus,  or  frost-bearer,  335. 
Crystallization,  310. 
Crystallogenic  attraction,  25. 
Crystals  conduct  heat,  244. 

forms  of,  24. 
Cubical  expansion,  217. 
Currents,  atmospheric,  258. 

in  gases,  256. 

in  the  ocean,  254. 
Curves,  magnetic,  628-630. 

paraboloid,  392. 
Curvilinear  motion.  842. 
Cyclones  or  hurricanes,  265. 
Cylinder  electrical  machine,  677. 


I). 


DAGUERREOTYPES,  how  formed,  486. 

taken  by  electrical  light,  750. 
Dark  lines  in  spectrum,  42b-428. 
Day  and  night,  867. 
Day,  solar,  854. 

sidereal,  854. 
Davy's  safety  lamp,  245. 
Dead  centre,  354. 
Declination  of  the  needle,  638. 
Declination  of  the  sun,  871,  931. 

effect  of  on  climate,  934. 
Decomposition  of  light,  419. 

of  salts,  756. 

of  water,  755. 
Defects  of  the  eye,  458-460. 
Deflagration,  751. 

Density  does  not  imply  hardness,  28. 
Density  of  air  at  different  altitudes,  139. 

of  gases  and  vapors,  229. 

of  planets,  with  table,  821. 
Descent  on  inclined  planes,  55. 


Descent,  perpendicular,  of  pendulums,  61. 
Depression  of  mercury  in  tubes,  38. 
Determination  of  reflective  power,  294. 
Dew,  343. 
Dew-point,  328. 
Diamagnetism,  783. 
Diathermancy,  298. 

causes  which  modify,  299. 

of  the  air,  300. 
Dielectrics,  670. 
Difference  between  musical  sounds  and  noises, 

588. 

Difference    between  static  and  dynamic    elec- 
tricity, 744. 

Differential  thermometer,  283. 
Diffraction,  511. 
Diffraction  fringes,  511. 
;  Dioptrics,  definitions,  394. 
|  Dipping  needle,  642. 

its  position  in  different  places,  643. 
Direction  in  which  objects  are  seen,  369. 
Direction  of  force,  54. 

of  gravity,  53. 

Directive  action  of  the  earth  and  of  magnets,  636. 
Discharge,  electrical,  of  the  condenser,  695. 
Discharger,  electrical,  695. 

universal,  697. 
Discharging-rod,  696. 
Discovery  of  electricity,  649. 

of  electro-magnetism,  760. 

of  galvanism,  accidental,  722. 
Disguised  electricity,  690. 
Dispersion  of  light.  364. 
i  Distance  calculated  by  sound,  537. 

estimation  of,  466. 

of  distinct  vision,  455. 
I  Distance  between  heavenly  bodies,  798. 
!  Diving-bell,  141. 
1  Divisibility,  11. 

Double  refraction,  polarization  by,~520. 
Downward  pressure  of  ;air,  126. 

of  liquids,  101. 
Dry-piles,  voltaic,  728.      < 
Ductility,  30. 

Dui'ation  of  visual  impressions,  470. 
Dynamical  electricity,  721. 
Dynamics,  54. 

E. 

EAR,  of  animals,  558. 

trumpet,  558. 
Earth  as  a  magnet,  633. 

as  viewed  from  Mercury,  836. 

at  the  equinoxes,  870. 

at  the  solstices,  869. 

circuit,  778. 

directive  action  of,  763. 

drawn  toward  falling  bodies,  52. 

figure  of,  50,  850. 

the  reservoir  of  electricity,  660. 

motion  of  the  water  of,  912. 
Earth's  axis,  nutation  of.  939. 

magnetism,  action  of  illustrated  by  mag- 
nets, 641. 

cause  of,  780. 

periodic  revolution,  815. 

rotation,  effect  of  upon  gravity,  51. 

effect  of  upon  winds,  262, 

satellite  or  moon,  826. 

surface,  convexity  of,  889. 
Ebullition,  316. 

laws  governing,  317. 

(see  boiling-point.) 
Echo,  547. 

tone  changed  by,  547. 
Echoes,  multiple,  548. 
Eclipses,  direction  in  which  they  come  on,  898. 

duration  of,  902. 

general  effects  of  total  of  the  sun,  903. 

of  the  stars,  909. 

position  of  sun,  earth,  and  moon  when  they 
occur,  896. 


31 


482 


INDEX. 


Eclipses,  the  number  of  in  any  one  year,  904. 

they  are  either  total,  partial,  or  annular,  897. 

total  and  annular  of  the  sun,  901. 

total  of  the  moon  and  partial  of  the  sun, 
899. 

why  there  are  not  more  solar  than  lunar, 
908. 

why  not  more  frequent,  905. 

of  Jupiter's  moons,  910. 

of  Saturn's  moons,  911. 
Ecliptic,  847. 

axis  of,  847. 

inclination  of  the  orbits  of  the  planets  to 
the  plane  of,  with  table,  849. 

its  intersection  with  the  equinoctial,  877. 

obliquity  of.  848. 

poles  of,  847. 

Ecliptic  limits,  solar  and  lunar,  907. 
Eel,  electrical,  790. 
Effects  of  accumulated  electricity,  704-708. 

chemical,  708. 

heating,  706. 

mechanical,  707. 

physiological,  705. 
Elastic  balls  transmit  shocks,  65. 
Elasticity,  26. 

limits  of,  26. 

of  air,  127. 

of  cords  and  wires,  570. 

of  flexure,  tension,  and  torsion,  26. 

of  gases,  147-150,  226. 

of  liquids,  89. 
Electric  battery,  699. 
Electric  currents,  action  of  magnets  upon,  769. 

action  of  upon  magnetic  needles,  761. 

attraction  of,  shown  by  oscillating  spiral, 
To8. 

difference  between  intensity  and  quantity 
of,  732. 

induced  by  other  currents,  784. 

mutual  action  of,  767. 

of  the  pile,  730. 

resistance  to,  742. 
Electric  light,  746. 

in  a  vacuum,  689. 

influences  the  magnet,  748. 

properties  of,  750. 
Electric  spark,  684,  700. 

color  of,  701. 

difference  between  positive  and  negative, 

702. 

Electric  telegraph,  776-778. 
Electrical  animals,  790. 
Electrical  attraction  and  repulsion,  656. 

attraction  and  repulsion,  laws  of,  657. 

eel,  790. 

excitement,  sources  of,  650. 

pendulum,  652. 

tension,  662. 
Electrical  machines,  676-683. 

precautions  in  using,  681. 

use  of,  679. 

Electrical  blasting,  706. 
Electrical  blow-pipe,  688. 

chime,  686. 

condensers,  690-694. 

egg,  689. 

experiments  illustrating  attraction  and  re- 
pulsion, 684-689. 

helix,  770,  771. 

induction,  668-671. 

puppets,  685. 

square,  703. 

wheel,  687. 
Electricity,  accumulated,  690-695. 

accumulated  only  on  the  surface  of  bodies, 
663. 

atmospheric,  709-719. 

conductors  of,  658. 

decomposition  by,  752. 

definition  of,  648. 

discharge  of,  694,  695. 

discovery  of,  649. 


Electricity,  disguised  or  latent,  690. 

distribution  dependent  on  form,  665. 

dynamical,  721.      . 

dynamical,  chemical  effects  of.  752-757. 

dynamical,  magnetic  effects  of,  759. 

dynamical,  physical  effects  of,  746-751. 

earth  a  reservoir  of,  660. 

Franklin's  experiment  with,  709. 

floating  currents  of,  767. 

frictional,  648-653. 

from  all  sources  identical,  744. 

from  steam,  682. 

galvanic,  725. 

heating,  effects  of,  706-751. 

illuminating  effects  of,  746. 

induction  of,  668-671. 

light  and  heat,  3. 

loss  of,  in  excited  bodies,  667. 

magneto,  781. 

measurement  of  quantity  of,  in  machines, 
680. 

mechanical  effects  of,  707. 

of  animals,  790. 

of  plants,  791. 

physiological  effects  of,  705,  758. 

positive  and  negative,  653. 

quantity  necessary  for  decomposition,  757. 

relation  between  and  magnetism,  759. 

resides  on  surfaces  of  bodies,  663. 

secondary  currents  of,  784. 

sources  of,  650. 

statical,  648. 

statical  and  dynamical,  difference  between, 
744. 

statical,  chemical  effects  of,  708. 

theories  of,  654,  655,  724,  725. 

thermo.  787. 

the  two  fluids  of,  separated  and  obtained, 
669. 

two  kinds  of,  653. 

velocity  of,  716. 

vitreous  and  resinous,  653. 

voltaic,  725. 
Electrodes,  727. 

shape  of  carbon,  749. 
Electro-chemical  decomposition,  752. 

theory,  725. 

Electro-dynamic  force,  exerted  in  a  tangential  di- 
rection, 765. 

Electro-dynamic  induction,  779. 
Electro-dynamics,  759. 
Electro-gilding  and  electro-plating,  754. 
Electro-magnetism,  759. 

Ersted's  discovery  relating  to,  760. 
Electro-magnets,  773. 

Electro-magnetic  force  exerted  in  a  tangential 
direction,  765. 

suspension  of  bodies  by,  without  contact. 
774. 

theory  of,  Ampere's,  766. 

utilization  of,  775. 
Electrometers,  672. 

gold-leaf,  674. 

method  of  using  the  gold-leaf,  675. 

quadrant,  673. 
Electro-motive  force,  741. 
Electro-positive  and  electro-negative,  731. 
Electrophorus,  676. 
Electroscope,  652. 

Bohnenberger's  dry-pile,  737. 
Electrotyping,  753. 

method  of  depositing  the  metal  upon  the 
mould,  753. 

preparing  the  mould,  753. 
Elements,  simple,  1. 
Emission  power  of  bodies,  296. 

causes  modifying,  297. 
Endless  screw,  86. 
Engine,  fire,  186. 

high  pressure  steain,  349. 

low  pressure  steam,  354. 
Eolipile,  346. 
Equilibrium,  40. 


INDEX. 


483 


Equilibrium,  conditions  of  in  liquids,  94. 

electrical,  655. 

neutral,  stable,  and  unstable,  40. 

neutral  and  stable  illustrated,  42. 

of  heat,  203. 

of  liquid*  in  communicating  vessels,  94. 

of  liquids  of  different  densities,  117. 

of  the  lever,  66. 

on  inclined  plane,  83. 
Equinoctial,  875. 

intersection  of  with  the  ecliptic,  877. 
Equinoxes,  870. 

precession  of,  875. 
ErstecTs  discovery,  760. 
Escape  of  liquids  through  orifices,  152. 
Essential  properties  of  matter,  9, 10. 
Estimation  of  distance  and  magnitude  of  ob- 
jects, 466. 

of  distance  by  sound,  537. 
Evaporation,  322. 

causes  influencing,  326. 

cold  produced  by,  335,  343. 

freezing  by,  335. 

in  a  vacuum,  323. 

under  pressure,  324. 
Expansibility,  13. 
Expansion  of  gases,  226. 

relation  of  to  compressibility,  228. 

laws  of,  227. 
Expansion  of  liquids,  220. 

amount  of,  221 . 

beneficial  effects  of  unequal,  in  water,  224. 

different  in  different  liquids,  for  same  heat, 

table  of  for  different  liquids,  222. 

water  an  exception  to  the  law.  223. 
Expansion  of  solids,  215. 

absolute  and  relative,  219. 

amount  of,  215. 

co-efficient  of,  cubical  and  lineal,  216. 

cubical,  217. 

cubical  and  lineal,  relation  between,  218. 

force  exerted  by,  215. 

linear,  215. 

ratio  of  increases  with  the  temperature, 
221. 

table  of,  for  different  solids,  219. 
Explosion  of  steam-boilers,  330. 
Extension,  9. 
Extent  of  space,  795. 
Extremes  of  temperature,  207. 
Eye,  adjustability  of  to  different  distances,  445. 

inversion  of  images  in,  449. 

lachrymal  or  tear  gland,  and  eyelid,  444. 

means  of  adjusting  and  holding,  442. 

optic  axis  of,  446. 

similarity  between  and    camera  obscura. 
440. 

structure  of  its  interior.  443. 

the  pupil,  method  of  adjusting,  441. 
Eye-glasses,  474. 
Eye-piece,  477,  488,  489. 


F. 


FAHRENHEIT'S  thermometer,  272. 

Fall  of  light  bodies,  129. 

Falling  bodies,  accelerated  velocity  of,  56. 

laws  of,  55. 
Falling  body,  space  described  by,  55. 

table  of  intervals  and  spaces,  55. 
Fire-engine,  186. 

where  first  employed,  186. 
Firmament,  951. 
Fixed  lines  in  spectra,  426-428. 

pulley,  75. 

stars,  940-951. 
Flame,  structure  of,  344. 
Flexibility  and  pliability,  31. 
Flotation,  principles  of,  118. 
Fluidify,  cause  of,  87. 
Fluids,  austral  and  boreal,  633. 


Fluids,  elastic.  119. 

electrical,  669. 

magnetic,  625. 

theory  of  single  electrical,  655. 

theory  of  two  electrical,  654. 

the  term  fluid,  655. 

viscid,  heating  of,  253. 
Flow  of  liquids,  153-155. 

in  pipes,  theoretical  and  actual,  153. 

through  orifices  at  different  depths,  155. 

velocity  of   discharge  as  square  root  of 
head,  155. 

of  rivers,  156. 

velocity  of,  157. 
Fly-wheel,  use  of,  354. 
Foci  of  concave  mirrors,  for  divergent  rays,  389. 

for  parallel  and  convergent  rays,  388. 

conjugate,  properties  of,  389. 
Focus,  virtual,  for  converging  rays,  388. 
Fog-bows,  486. 
Force,  coercitive  of  magnets,  627. 

directive  of  magnets,  636. 

distribution  of,  in  magnets,  605. 

electro-motive,  741. 

formative  in  nature,  24. 

origin  of,  20. 

unit  of,  54. 
Forces,  20. 

centrifugal  and  centripital,  843. 

composition  of,  58. 

measure  of,  54. 

molecular,  22. 

parallelogram  of,  58. 

propositions  in  regard  to,  54. 

represented  by  lines,  54. 

resolution  of,  58. 
Forces  and  resistances,  66. 
Forcing-pumps,  180-185. 

plunger.  180. 

rotary,  75-177. 

Formative  force  in  nature,  24. 
Forms  of  crystals,  25. 

of  lenses,  405. 

of  mirrors,  367. 

Formulae  relating  to  combination  of  mechani- 
cal powers,  86. 

relating  to  compound  levers,  69. 

compound  wheel  and  axle,  74. 

conversion  of  thermometric  scales,  273. 

falling  and  rising  bodies,  55. 

hydrostatic  press,  109. 

inclined  plane,  83. 

levers.  66-68. 

pressure  of  liquids,  108, 133. 

pulleys,  75-82. 

the  screw,  84. 

specific  gravity,  115. 

velocity  of  discharge  of  liquids,  155. 

the  wedge,  85. 

the  wheel  and  axle,  71. 
Fountain  and  vertical  jets  of  water,  195. 
Fountain,  condensed  air,  149. 

expansion,  131. 

Hiero's,  196. 

intermittent,  197. 

vacuum,  145. 
Franklin's  kite.  709. 

harmonicon,  563. 

pulse-glass,  319. 
Freezing  by  evaporation,  335. 
Freezing  mixtures,  309. 
Freezing  point,  272. 

fixing  it  on  thermometers,  276. 
Freezing  a  wanning  process,  306. 
Friction  between  liquids  and  solids,  153. 

electricity  excited  by,  653. 

heat  produced  by,  212. 

in  rivers,  156. 
Frictional  electricity,  648. 

Fringes,  diffraction,  caused  by  interference,  511. 
Frost-bearer,  335. 
Fulcrum,  66. 
Furnaces,  hot-air,  256. 


4S4 


INDEX. 


Fusion,  always  gradual,  306. 
latent  heat  of,  303. 
laws  and  beat  of,  304. 
peculiar  in  some  solids,  305. 


G. 

GALAXY,  or  the  milky  way,  948. 
Galileo's  discoveries,  805. 
Galvanic  battery,  725. 
Galvanism,  721. 

discovery  of,  722. 

Galvani's  explanation  of,  723. 

Volta's  contact  theory  of,  724. 
Galvanometers  or  multipliers,  762. 
Gamut,  596. 
Gas,  illuminating,   344. 

ignited  by  electricity,  683. 
Gases  and  vapors,  119. 

capacity  for  heat,  236. 

compressibility  of,  127. 

compression  of,  diminishes  capacity  for 
heat,  236. 

conductibility  of,  for  heat,  249. 

density  of,  229. 

expansion  of,  120,  226. 

impenetrability  of,  125. 

laws  of  expansion  of,  227. 

Mariotte's  law  of  elastic  force  of,  147. 

mechanical  conditions  of,  121. 

molecular  force  of  repulsion  of,  22,  23,  88, 
91,  119. 

permanent,  incoercible,  120. 

simple  or  compound,  120. 

specific  heat  of,  235. 

tension  of,  affected  by  temperature,  323-325. 

they  transmit  pressure,  121. 
Gas-jets,  musical  notes  of,  574. 
Glass,  burning,  301. 

night,  482. 

object-glass  and  eye-glass,  481. 

opera,  481. 
Globes,  celestial,  938. 

terrestrial,  937. 

Graduation  of  thermometers,  276. 
Gravity,  39. 

affected  by  the  earth's  rotation,  51. 

affected  by  the  shape  of  the  earth,  50. 

cause  of  weight,  39. 

centre  of,  40. 

centre  of,  in  man,  46. 

centre  of,  in  vehicles,  45. 

centre  of,  of  the  solar  system,  845. 

direction  of,  53. 

law  of  intensity  of,  47. 

tabular  statement  of  the  law,  47. 

varies  with  altitude,  48. 

varies  with  latitude,  50. 

varies  with  depression  below  level  of  the 
sea,  49. 

specific,  method  of  finding,  115,  116. 
Guage,  rain,  339. 
Gridiron,  pendulum,  60. 
Grove's  battery,  738. 
Gulf  stream,  254. 


H. 

HAIL,  338. 

Halos,  436. 

Hand-truck,  a  variety  of  lever,  69. 

Hardening,  33. 

by  hammering,  33. 

by  heating  and  cooling,  .33. 
Hardness,  28. 
Harmonicon,  563. 
Harmonies,  limit  of,  595. 

most  pleasing,  594. 

the  principal,  593. 
Harmony.  592. 
Heat,  action  of  on  matter,  202. 


Heat  and  cold  relative  terms,  199. 

and  light,  analogy,  358. 

applied  to  warming     apartments 

consumed  in  expanding  the  air, 

atmospheric  electricity,  a  source  of,  209. 

cause  of  in  animals,  213. 

causes  which  modify  the  reflective,  absorb- 
ent, and  emission  power  for,  297. 

change  of  state  in  bodies  caused  by,  303. 

chemical  effects  of,  202. 

combustion,  a  source  of,  211. 

conduction  of,  240-252. 

convection  of,  in  gases  256. 

convection  of,  in  fluids,  253-255. 

definition  of,  198. 

developed  by  solidification,  303. 

distribution  of,  202. 

effects  of  on  magnets,  608. 

equilibrium  of,  203. 

expansion  of  gases  by,  226. 

expansion  of  liquids  by,  220. 

expansion  of  solids  by,  215. 

general  effects  of,  202. 

intensity  of  in  solar  spectrum,  420. 

its  effects  on  organic  life,  202. 

latent,  303. 

light  and  electricity,  3. 

luminous  and  obscure,  204. 

mechanical  sources  of,  212. 

modes  of  communication  of,  239. 

nature  of,  201. 

of  chemical  action,  210,  211,  344. 

of  compression,  212,  236. 

of  friction,  212. 

of  fusion,  304. 

of  percussion,  212. 

of  plants,  213. 

of  static  electricity,  706. 

of  voltaic  arch,  751. 

of  voltaic  currents,  751. 

origin  of  terrestrial,  208. 

physiological  sources  of,  213. 

polarization  of,  302. 

quantity  and  intensity,  difference  between, 
214. 

quantity  emitted  by  the  sun,  206. 

radiation  of,  284-289. 

reflection  of,  2SO-294. 

refraction  of,  301.  419,  420. 

relation  of  to  cold,  199. 

repellent  force  of,  202. 

sensible,  303,  315. 

sensible  of  steam,  315. 

solar  radiation,  205. 

specific,  231. 

specific  affected  by  change  of  state.  238. 

specific  of  pases,  2&5. 

specific  of  water,  effect  on  climate,  234. 

standard  of  specific,  233. 

theories  respecting,  201. 

transference  of,  203. 

transmission  of  radiant,  298. 

unit  of,  232. 

universal  radiation  of,  289. 

velocity  of,  509. 

Heat  and  light  of  the  planets,  with  table,  823. 
Heating  buildings  by  hot  air,  256. 

by  hot  water,  255. 

by  steam,  257. 
Heavenly  bodies,  different  classes  of,  794. 

distances  between,  798. 

magnitude  of,  796. 

orbital  motions  of,  799. 

the  number  of,  797,  949. 

velocity  of.  800. 

Height  measured  by  barometer,  135. 
Helix  electrical,  single,  770. 

double,  771. 

magnetizing  by,  772. 
Hemisphere,  invisible,  938. 

visible,  938. 
Hiero's  fountain,  196. 
High-pressure  engine,  349. 


INDEX. 


485 


High-pressure  stetim,  3:34. 

Horizon,  crimson  appearance  of,  400. 

celestial,  938. 

sensible,  938. 
Horse-shoe  magnets,  610. 
Hot  air  furnaces,  256. 
Hot  water  apparatus,  255. 
House's  telegraph,  777. 
Humidity  of  the  air,  343. 
Hurricanes,  265. 

Hydraulic  and  hydro-pneumatic  machines,   im- 
portance of,  197. 
Hydraulic  ram.  172. 
Hydraulics,  definition  of,  151, 
Hydro-dynamics,  151. 
Hydro-electric  machine,  682. 
Hydrogen,  density  of,  229. 
Hydrometers,  110. 
Hydrostatic  bellows,  111. 

paradox,  107. 

press,  example  and  formulae,  109. 

pressure  in  mountains,  112. 
Hydrostatics,  87. 
Hygrometer  or  moisture-bearer,  343. 


ICE,  beneficial  effects  of  being  lighter  than  water, 

224. 

lighting  gas  with,  706. 
why  it  does  not  acquire  great  thickness, 

why  lighter  than  water,  224. 
Illumination  of  railways,  392. 
Illumination,  best  materials  for,  344. 

sufficiency  of,  452. 
Image?  formed  by  concave  lenses,  418. 

by  concave  reflectors,  387. 

by  concave  reflectors,  when  the  object  is 
beyond  the  centre  of  curvature,  393. 

by  convex  lenses,  when  the  object  is  twice 
the  focal  distance,  413. 

by  convex  lenses,  when  the  object  is  at 
more  or  less  than  twice  the  focal  dis- 
tance, 414,  415. 

by  convex  reflectors.  384,  385. 

by  plane  reflectors,  375. 
Images,  inversion  of  in  the  eye,  449. 

multiplicity  of,  376. 

size  of  on  the  retina,  457. 

virtual,  375. 
Impenetrability,  10. 

of  gases,  125. 
Imponderables,  3. 
Incidence,  angle  of,  57. 
Inclination  of  orbits  of  planets  to  plane  of  ecliptic, 

table  of,'  849. 
Inclination  of  moon's  orbit  to  plane  of  ecliptic, 

883. 

Inclination,  polar,  of  the  planets,  878. 
Inclined  plane,  83. 

conditions  of  equilibrium,  83. 

example  and  formulae,  S3. 

its  combination  with  the  other  mechanical 
powers,  86. 

example  and  formulae,  86. 

the  screw,  a  modification  of,  84. 

example  and  formula},  84. 

the  wedge,  a  modification  of,  85. 

form  u  lie.  85. 
Indestructibility.  17. 
Index  of  reft  action.  394. 
Induced  curronis,  784. 

different  orders  of,  785. 

properties  of,  786. 
Induction,  electro-dynamic,  779. 

explanation  of*  electrical,  670. 

magnetic,  illustrated  by  a  series  of  rings, 
617. 

magnetic,  without  contact,  620. 

of  electricity,  668-671. 


Induction  of  magnetism,  616^620. 

Inductive  power  of  the  earth's  magnetism,  624. 

Inertia,  16. 

Influence  of  the  earth's  figure  on  gravity,  50. 

of  the  earth  on  its  waters,  915. 

relative  of  sun  and  moon  on  the  tides,  922. 
Influence  of  the  sun,  811.  ^ — ' 

upon  tides,  919. 
Insulation  and  insulators,  659. 
Insulating  stool,  684. 
Intensity,  conditions  of,  of  light,  527. 

in  electricity,  732,  733. 

of  force,  54. 

of  gravity,  47. 

of  light  at  different  distances,  528,  529. 

of  light,  increases  with  angle  of  incidence. 
374,  527. 

of  light,  reflected,  374. 

of  luminous,  calorific,  and  chemical  rays, 

of  many  couples,  733. 

of  radiant  heat,  286. 

of  sound,  555. 

of  sound,  causes  which  modify,  556. 

of  sound  in  tubes,  557. 
Interference  colors,  507. 
Interference  of  light,  503. 

demonstration  of,  505. 

fringes  caused  by,  511. 

laws  of,  506. 

non-interference,  504. 

of  sound,  564. 

of  sound  waves,  568. 

of  waves  of  liquid  in  an  ellipse,  566. 
Intermittent  fountain,  197. 

springs,  99,  189. 

Intersection  of  the  ecliptic  and  equinoctial,  877. 
Interstices  between  atoms  and  molecules,  35. 
Iris,  441. 

Iron,  how  made  magnetic,  604. 
Iron  ships,  the  Great  Eastern,  118. 
Irradiation,  465. 


J. 

JAR,  Leyden,  692. 
Jets  of  water,  195. 
Jupiter,  836. 
Jupiter's  belts,  836. 

length  of  days  and  nights,  878. 

seasons,  878. 

Jupiter's  satellites  or  moons,  827. 

eclipses  of,  910. 

dimensions,  distances,  and  periodic  times 
of,  827. 


K. 

KALEIDOSCOPE:,  377. 
Keeper  of  a  magnet,  610. 
Kepler's  discoveries  and  laws,  804. 


LANTERN,  magic,  478. 
Latent  electricity,  690. 
Latent  heat,  303. 

and  sensible,  of  steam,  315. 

of  evaporation,  313. 

of  fusion,  303. 

of  steam,  314. 

of  water  graduates  changes   of  tempera- 
ture, 308. 

Lateral  pressure  of  fluids,  to  what  proportioned, 
104. 

total  of,  on  walls  of  a  vessel,  105. 
Latitude  found  by  the  north  star,  889. 

celestial  and  terrestrial,  935. 

circles  of  celestial,  938. 
Laws  Of  cooling  by  radiation,  286. 


486 


INDEX. 


Laws  of  distribution  of  attraction  in   magnets, 
606,  607. 

determining  the  force  of  voltaic  currents, 
743. 

electrical  attraction  and  repulsion,  657. 

electrical  induction,  669. 

evaporation,  323. 

expansion  of  gases,  227. 

falling  and  rising  bodies,  55. 

intensity  of  gravity.  47. 

intensity  of  light,  527,  529. 

intensity  of  radiation  of  heat,  286. 

interference  of  light,  506. 

Kepler,  804. 

liquefaction  and  solidification,  304. 

magnetic  attraction  and  repulsion,  626. 

oscillation  of  pendulums,  61. 

projectiles,  62. 

reflection  of  heat,  291. 

reflection  of  light,  368. 

reflected  motion,  57. 

refraction  of  light,  395. 

refraction  of  sound,  582. 

the  vibration  of  cords,  572. 
Length  of  luminous  waves  508,  509. 
Lenses  and  prisms,  405. 

analogous  effects  of,  411. 
Lenses,  aberration  of,  412. 

achromatic  combination  of,  438. 

conjugate  foci  of,  409,  410. 

convergent  and  divergent,  405. 

definitions  relating  to,  408. 

varieties  of,  405. 
Lenses  concave,  417/ 

effects  of,  on  rays  of  light,  417. 

images  formed  by,  418. 

convex,  action  of  on  light,  408. 

images  formed  by,  413-415. 

magnifying  power  of,  476. 

optical  centre  of,  408. 

plano-convex,  412. 

spherical,  effect  of  on  a  ray  of  light,  407. 
Level,  water,  96. 

spirit,  97. 
Lever,  conditions  of  equilibrium  of,  66. 

definition  of,  66. 

of  the  first  class,  66. 

example  and  formulae,  66. 

of  the  second  class,  67. 

example,  67. 

of  the  third  class,  68. 

example,  68. 

illustrated  by  limbs  of  animals,  70. 

compound,  69. 

example  and  formulae,  69. 
Leyden  jar,  692. 

charging  of.  693. 

discharge  of.  695. 

disruptive  discharge  of,  694. 

electricity  of  resides  on  the  glass,  698. 

limit  of  charge  of,  694. 

slow  discharge  of,  720. 
Libration  of  the  moon,  858. 
Light,  absorption  of,  361,  364. 

action  of  tourmaline  on,  514. 

and  heat,  analogy  of,  358. 

and  heat  by  chemical  action,  210,  211,  344. 

artificial,  357. 

cause  of  refraction  of,  396. 

cause  of  waves  of,  510. 

changed  by  polarization,  513-520. 

colors  of,  419. 

color  of  dependent  on  length  of  waves, 
502. 

definition  of,  355. 

dispersion  of,  364. 

direction  of  vibrations  of,  500. 

double  reflection  of  by  mirrors,  398. 

double  refraction  of,  519. 

electricity  a  source  of,  357. 

emitted  from  every  point  of  visible  bodies, 

heat  and  electricity  are  forces  in  nature,  3. 


Light  in  a  homogeneous  medium,  360. 
influenced  by  magnetism,  748. 
interference  colors  of,  507. 
intensity  of  at  different  distances,  529. 
intensity  of    dependent  upon  conditions, 

interference  and  non-interference  of,  503- 

506. 

internal  reflection  of,  398. 
laws  of  refraction  of,  395. 
length  of  vibrations  or  waves  of,  509. 
moves  in  straight  lines,  363. 
nature  of,  356. 
parallel  rays  of,  how  affected  by  a  drop  of 

water,  435. 


polarization  of,  512-521. 

properties  of,  364. 

rays,  pencils,  and  beams  of,  362. 

reflection  of,  370-393. 

reflection  of,  total,  399. 

recomposition  of  in  several  ways,  431. 

refraction  of,  by  dense  media,  401. 

refracted  by  parallel  strata,  397. 

refraction  by  prisms,  406,  419. 

relation  of  different  bodies  to,  359. 

sensations  of,  excited  by  other  causes,  473. 

sources  of,  357. 

theories  of,  356,  499. 

velocity  of,  526. 

waves  of,  499-510. 

white,  composition  of,  431. 
Light  and  heat  of  planets,  with  table,  823. 
Light-houses,  416. 

revolving,  416. 
Lightning,  714. 

classes  of,  715. 

Franklin's  experiment  with,  709. 

identity  of  and  electricity,  709. 

liability  of  being  struck *by,  718. 

means  of  safety  from,  718. 

return  shock  of,  717. 

velocity  of.  716. 
Lightning  rods,  718. 

how  to  render  them  effective,  718. 
Limbs  of  animals  levers  of  the  third  class,  70. 
Limits  of  elasticity,  26. 

of  perceptible  sounds,  590. 
Linear  expansion,  215. 

co-efficient  of,  216. 

laws  of,  215. 
Liquefaction  and  solidifaction,  304. 

always  gradual,  306. 

laws  of,  304. 
Liquids,  ascent  of,  in  capillary  tubes,  38. 

cohesion  in.  90. 

compressibility  of,  89. 

convection  of  heat  in,  253-255. 

direction  of  pressure  of.  92. 

downward  pressure  of,  101. 

equilibrium  of  in  communicating  vessels, 
94. 

equilibrium  of  different  densities  in  com- 
municating vessels,  102. 

expansion  of,  220-224. 

flow  of  obstructed  by  sharp  angles,  190. 

friction  between  and  solids,  153,  156. 

heat  in,  not  equalized  by  conduction,  247. 

lateral  pressure  of,  diminished  by  motion, 
194. 

mobility  of,  88. 

mobility,  cause  of,  88. 

non-conductibility  of,    shown   by  experi- 
ment, 248. 

of  unequal  densities  seek  different  levels 
in  containing  vessels,  117. 

practical  use  of  transmitting  pressure  by, 
108. 

pressure  of,  not  in  proportion  to  quantity, 
but  height,  93. 

pressure  of,  in  proportion  to  height  and 
base,  103. 

pressure  of,  on  sides  of  vessels,  104. 


INDEX. 


487 


Liquids,  pressure,  total,  on  sides  of  vessels,  105. 
pressure,   total,   on  sides  and  bottom  of 

vessels,  106. 
spheroidal  state  of,  331. 
specific  gravity  of,  115,  116. 
tendency  of  to  seek  a  level,  shown  by  aque- 
ducts, 98. 
transmit  pressure  equally  in  all  directions, 

upward  pressure  of,  equal   to  downward 
pressure,  100. 

vary  in  fluidity,  88. 

velocity  of  discharge  of,  155. 

volatile  and  fixed.  312. 
Lodestone,  599. 

magnetic  manifestations  of,  600. 

method  of  making  magnets  with,  604. 

north  and  south  poles  of,  600. 
Longitude  in  the  heavens,  876. 
'   angle  of,  938. 

ct'lestial,  936. 
Long-sightedness.  458. 

caused  by  defective  form  of  eye-ball,  459. 

of  old  people,  460. 
Looming,  403. 
Low-pressure  steam-engine,  354. 

illustration  of  the  principle  of,  332,  333. 


M.     ' 

MACHINE,  motor,  power,  and  weight,  66. 
Machines,  electric,  676-683. 

for  elevating  water,  164-185. 

importance  of  hydraulic  and  hydro-pneu- 
matic. 197. 
Magic  lantern,  478. 

Magnetic  attraction  not  intercepted,  631. 
Magnetic  attraction  and  repulsion,  625. 

at  different  distances,  607. 

distribution  of,  606. 

laws  of,  626. 
Magnetic  curves.  628-630. 

batteries,  610. 

dip  of  needle,  640. 

electricity,  781. 

fluids,  625. 

induction,  616-624. 

intensity  varies,  645. 

manifestations  of  lodestone,  599, 600. 

meridian,  637 

needle.  635. 

polarity,  600. 

Magnetic  and  magnetized  bodies,  615. 
Magnetism  by  contact,  616. 

by  induction,  616. 

coercitive  force  of,  627. 

definition  of,  598. 

inductive  power  of  the  earth's,  646. 

relation  between  and  electricity,  759. 

terrestrial,  (533. 

terrestrial,  illustrated  by  action  of  magnets, 
641. 

two-fluid  theory  of,  625. 

utilization  of,  647. 
Magnetizing,  by  the  helix  and  electrical  current, 

method  of  by  bent  bars,  612. 

method  of  by  straight  bars,  613. 
Magneto-electric  machines,  782. 
Magneto-electricity,  781. 
Magnete,  armatures  of,  601. 

artificial,  603. 

bar,  609. 

both  poles  of  must  co-exist,  614. 

compound,  609. 

compound  horse-shoe,  610. 

deprived  of  power  by  heat,  608. 

directive  force  of,  636. 

directive  force  of,  simply  rotates  the  needle, 
636. 

distribution  of  force  in,  605. 

do  not  part  with  their  own  power,  621. 


magnets,  electro,  773. 

force  of  attraction  of.  at  different  distances, 
607. 

fully  mounted  lodestone,  602. 

law  of  distribution  of  attraction  in,  606. 

method  of  charging,  611-613. 

method  of  charging  horse-shoe,  612. 

method  of  making  artificial,  604. 

natural,  599. 

preservation  of,  632. 

unlike  poles  of  neutralize  each  other,  622, 

623. 

Magnifying  glasses,  476-479. 
Magnifying  power  of  lenses,  476. 
Magnitude  or  extension,  9. 

absolute  of  planets,  with  table,  818. 

of  heavenly  bodies,  796. 

relative  of  planets,  with  table,  819. 
Malleability,  29. 
Mariner's  compass,  644. 

discovery  of,  644. 

variations  of,  corrected  by  table,  644 
Mariner's  sextant,  380. 
Matter,  1. 

accessory  properties  of,  11-18. 

changes  in,  chemical  or  physical,  2. 

different  kinds  of,  1. 

essential  properties  of,  9,  10. 

properties  of,  general  or  specific,  6. 

spaces  between  atoms  of,  5,  35. 

the  three  states  of,  23. 

ultimate  constitution  of,  4. 
Mechanical  conditions  of  gases,  121. 
Mechanical  powers,  66-86. 
Medium,  luminiferous,  360. 
Mediums  of  sound,  532. 
Melody,  592. 
Melting  a  cooling  process,  306. 

and  freezing,  304. 

always  gradual,  306. 

peculiarities  of  in  some  solids,  305. 
Mercurial  thermometer,  272. 

limits  of,  280. 

Mercury,  depression  of,  in  tubes,  38. 
Mercury,  oscillations  of.  882. 
Mercury,  transits  of,  880. 

list  of,  for  the  present  century,  881. 
Meridian,  magnetic,  637. 

true,  637. 
Metals  conduct  electricity,  242. 

conduct  heat,  242. 
Microscopes,  476. 

compound,  477. 

object  glasses  of,  477. 

power  of,  476,  477. 

simple,  476. 

solar,  479. 

Microscopic  views,  479. 
Milky  way,  an  annular  nebula,  948. 
Mirage,  402. 
Mirrors  and  specula.  366. 

concave,  convex,  and  plane,  373. 

concave  reverse  of  convex,  386. 

conjugate  foci,  properties  of,  389. 

convex    spherical,   illustrated    by    plane 
381. 

deception  practiced  by,  378. 

foci  of  concave,  for  parallel  and  convergent 
rays,  388. 

foci  of  concave,  for  divergent  rays,  389. 

forms  of,  367. 

objects  reflected  double  the  size  of,  379. 

paraboloid,  392. 

spherical  aberration  of,  391. 
Mobility,  15. 

of  gases,  121. 

of  liquids,  88. 
Molecules,  5. 

spaces  between,  5,  35. 
Momentum,  21,  54. 

Moon,  as  seen  from  the  poles  and  equator  of 
the  earth,  884. 

dark  and  light  spots  of,  864. 


488 


INDEX. 


Mobil,  eccentricity  of  her  orbit,  864. 

her  actual  path,  859. 

her  libration  in  longitude  and  latitude,  858. 

her  light  compared  with  the  sun's,  864. 

her  motion  never  retrograde,  859. 

her  orbit  always  concave  toward  the  sun, 
860. 

her  path  around  the  sun,  855. 

her  phases,  862. 

inclination  of  her  orbit  to  plane  of  ecliptic, 
883. 

importance  of  the  phases  and  motions  of, 
86-2. 

motion  of,  826. 

rotation  of,  on  its  axis,  857. 

sidereal  and  synodic  revolution  of,  856. 

size  of,  864. 

view  of  the  earth  from,  861. 

weight  of,  864. 

why  her  dark  side  is  visible  near  conjunc- 
tion, 863. 

when  it  is  new  and/wW,  855. 

why  it  rises  later  every  day,  864. 
Morning  and  evening  star,  Venus,  851. 
Morse's  telegraph,  777. 
Motion  and  force,  si4. 
Motion,  absolute  and  relative,  15. 

accelerated,  retarded,  and  uniform,  54. 

centre  of,  of  the  solar  system,  845. 

compound  of  the  satellites,  825. 

curvilinear,  842. 

direct,  stationary,  and  retrograde  of  planets, 
891. 

of  projectiles,  62. 

of  the  stars,  940. 

of  the  waters  of  the  earth,  912. 

reflected,  57. 

resultant,  58. 

varieties  of,  54. 
Motions  of  the  primary  planets,  815-817. 

of  the  secondary  planets,  825-830. 

of  the  sun,  814. 
Motors,  66. 
Musical  scale,  596. 

formation  of.  597. 

Musical  sounds,  difference  between  and  noises, 
588. 

qualities  of,  589. 


N. 

NATURAL  PHILOSOPHY,  distinction  between  and 

chemistry,  8. 

Neap  and  spring  tides,  923-925. 
Near-sightedness,  458. 

caused  by  defective  form  of  eye-ball,  459. 
Nebulae,  946. 

classes  of,  947. 
Needle,  astatic,  634. 

declination  of,  638. 

dipping,  642. 

diurnal  and  other  variations  of,  639. 

inclination,  or  dip  of,  640. 

magnetic,  635. 

mariner's,  644. 

position  of  dipping,  in  different  parts  of 

the  earth,  643. 
Neptune's  satellites,  830. 
Neutral  equilibrium,  40,  42. 
Neutralization  of  magnetic  poles,  622. 

shown  by  Y-magnets,  623. 
Newton's  discovery,  806. 
Night-glass,  482. 
Nitric  acid  battery,  738. 
Nitrogen,  density  of,  229. 
Nodal  figures  and  lines.  580. ' 

how  delineated,  579. 
Nodal  points,  576. 

lines  of  plates,  578. 
Nodes.  879. 

retrograde  motion  of  the  moon's,  906. 
Noise,  588 


Notes,  musical,  absolute  number  of  vibrations 

corresponding  to  each,  597. 
Nutation  of  the  earth's  axis,  939. 
Nut-cracker  an  example  of  levers,  69. 


O. 


OBJECT-GLASSES  for  the  microscope,  477. 


for  telescopes,  488. 
iquity  of  the  ecliptic,  848. 
saltation  of  the  stars,  909. 


Oblic  .      . 

Occupation  of  the  stars,  909. 

Ocean,  currents  in,  254. 

Octave  in  music,  593. 

Opaque  bodies,  359. 

Opera-glasses,  481. 

Opposition  and  conjunction  of  planets,  890. 

Optic  angle,  447. 

Optical  axis,  446. 

Optical  centre  of  a  lens,  408. 

Optical  instruments.  474-498. 

camera  obscura,  439,  483,  484. 

camera  lucida,  485. 

magic  lantern,  478. 

microscope,  compound,  477. 

microscope,  simple,  476. 

microscope,  solar,  479. 

night-glasses,  482. 

opera-glasses,  481. 

spectacles,  475. 

stereomonoscope,  498. 

stereoscope,  496,  497. 

telescopes,  488-494. 

telestereoscope,  495. 

variety  and  principal  uses  of,  474. 
Optical  toys,  471. 
Optics,  definition,  355. 
Orbit  of  the  moon  always  concave  toward  the 

sun,  860. 

Orbital  motions  of  heavenly  bodies,  799. 
Orbits  of  comets,  832. 
Orbits  of  heavenly  bodies,  elliptical,  837. 

aphelion  and  perihelion  of,  839. 

eccentricity  of,  with  table,  838. 

plane  of,  846. 

radius  vector  of,  840. 
Organic  electricity,  789-791. 
Organs  of  voice  a  reed  instrument,  587. 
Orifices,  shape  of,  152. 
Oscillations  of  pendulums,  60,  61. 

laws  of,  61. 

Oscillations  of  the  planet  Mercury,  882. 
Overshot  wheel,  159. 
Oxygen,  density  of,  229. 


P. 


PARABOLIC  curve,  392. 

mirrors  and  reflectors,  392. 
Paradox,  hydrostatic,  107. 
Parallax  of  the  heavenly  bodies,  885-888. 

annual  of  the  stars,  885. 

diurnal,  886. 

effect  of  on  bodies,  887. 

importance  of  the  principles  of,  888 
Parallelogram  of  forces,  58. 
Pencils  of  light,  362. 

oblique,  390. 
Pendulum,  compensating,  60. 

electrical,  652. 

laws  of  oscillation  of,  61. 

scientific  uses  of,  61. 
Penumbra,  524. 

Percussion  a  source  of  heat,  212. 
Perihelion,  839. 

Periodic  time  of  heavenly  bodies,  815. 
Perpetual  revolution,  63. 
Phases  of  undulations  of  light,  503-506. 

of  sound,  564-5(i8. 

of  the  moon,  862. 
Philosophy  of  eclipses.  894-911. 

of  seasons,  867-871. 


INDEX. 


489 


Philosophy  of  tides,  912-925. 

of  transits,  879-881. 
Phosphorescence,  357. 
Photography,  487. 
Photometers,  528. 

Bunsen's,  Ritchie's,  Rumford's,  Silliman's, 

Physical  astronomy,  793. 

Physical  properties  of  winds,  267. 

Physics,  or  Natural  Philosophy,  8. 

Physics,  or  Natural  Philosophy  and  chemistry, 

distinction  between,  8. 

Physiological  effects  of  statical  or  frictional  elec- 
tricity, 705. 

of  dynamical  or  voltaic  electricity,  758. 
Pipes,  rapidity  of  water  discharged  from,  155. 

reed,  585. 

sound  from,  583. 

with  fixed  mouth-pieces,  584. 
Plane  glass,  refraction  by,  397,  398. 
Plane  of  meridian,  938. 

of  the  ecliptic,  inclination    of   orbits  to, 

of  the  equinoctial,  847. 
Planes,  inclined,  83-85. 

of  orbits,  846. 
Planets,  807. 

approximate  relative  distances  of,  809. 

difference   between  equatorial  and  polar 
diameters  of,  850. 

direct,  stationary,  and  retrogade,  motion 
of,  891. 

exterior  and  interior,  807. 

figure  or  form  of,  850. 

greatest  declination  of,  878. 

opposition  and  conjunction  of,  890. 

polar  inclination  of,  878. 

primary,  807,  815. 

relative  magnitude  of,.808. 

representation  of  the  motions  of,  810. 

secondary,  825-830. 

telescopic  views  of,  836. 

why  they  do  not  fall  to  the  sun,  844. 

width  of  zones  of,  878. 

(see  primary  planets). 

Plants  consume  carbonic  acid  and  supply  oxy- 
gen, 124. 

electricity  of,  791. 
Plate  electrical  machine,  678. 
Plates,  vibration  of,  577. 
Pliability,  31. 
Plumb-line,  53. 
Pneumatics,  definitions,  119. 
Polar  inclination  and  seasons  of  different  planets, 

with  table,  878. 

Polarity  of  magnets,  600,  614,  619,  622,  623. 
Polarity  of  the  pile,  729. 

Ersted's  discovery  of,  760. 
Polarization  and  transfer  of  elements,  756. 
Polari  scope,  515. 
Polarization  of  light,  513-521. 

by  reflection,  515. 

by  refraction,  519. 

by  double  refraction,  520. 

by  transmission,  513. 

partial,  518. 

plane,  516. 

useful  applications  of,  521. 
Poles,  celestial,  938. 

in  physics,  512. 

of  magnets,  600. 

arrangement  of  in  star-shaped  bodies,  618. 

both  must  co-exist  in  every  magnet,  614. 

two  sets  of,  619. 

unlike  neutralize  each  other,  622. 

of  the  earth,  when  the  sun  shines  upon 
them,  933. 

of  the  ecliptic,  847. 
Polyrama,  480. 
Porosity,  14. 

Poros,  physical  and  sensible,  14. 
Positive  and  negative  in  electricity,  653. 

in  magnetism,  605. 


Power,  66. 

of  points,  electrical,  666. 

of  steam,  202,  329,  334,  349,  354. 

relation  of  to  weight,  66. 
Press,  hydrostatic,  109. 
Pressure,  atmospheric,  126. 

action  of  barometers  dependent  on,  134. 

action  of  suction  pumps  dependent  on,  132, 
173, 174. 

amount  of  on  the  human  body,  126. 

animal  respiration  dependent  on,  146. 

equal  in  all  directions,  130. 

illustrated  in  a  vacuum,  129. 

it  varies  with  altitude,  132. 

shown  by  currents  of  air,  143. 

shown  by  hollow  hemispheres,  130. 

shown  by  inverted  tumbler  of  water,  142. 

shown  by  tubes  and  water,  144. 

shown  by  vacuum  fountain,  145. 

sustains     different     liquids    at    different 
heights,  133. 

varied  on  liquids  varies  the  boiling  point, 
318. 

varies  at  the  same  place,  133. 
Pressure  of  liquids,  92. 

bursting  a  cask  by,  110. 

downward  of,  101. 

downward    of  independent   of  shape    of 
vessel,  101. 

equal  in  all  directions,  92. 

not  in  proportion  to  quantity,  but  height, 
93,  107. 

on  walls  of  vessels,  104. 

on  walls  of  vessels,  total,  105. 

on  walls  and  bottom  of  vessels,  total,  106. 

transmitted  equally  in  all  directions,  92. 

upward  equal  to  downward,  100. 

use  of,  transmitted,  108,  109. 
Primary  colors,  419. 

planets,  815. 

absolute  magnitude  of,  with  table,  818. 

attraction  of,  with  table,  822. 

density  of,  with  table,  821. 

distance  of,  from  Sun,  with  table,  820. 

diurnal  revolution  of,  with  table,  817. 

light  and  heat  of,  with  table,  823. 

periodic  revolution  of,  with  table,  815. 

relative  niagnitude  of,  with  table,  819. 

telescopic  views  of,  836. 

velocity  of,  with  table,  816. 
Printing  telegraph,  777. 
Prisms,  405. 

refraction  by,  406,  419. 
Projectiles,  falling  of,  64. 

greatest  horizontal  range  of,  62. 

motion  of,  62. 

thrown  from  horizontal  guns,  64. 
Proof  plane,  664. 
Properties,  characteristic,  of  solids,  25-34. 

of  fluids  and  gases,  87. 

of  gases,  119-133. 

of  light,  364. 

of  liquids,  92-118. 

of  the  solar  spectrum,  420. 

of  matter,  essential,  9, 10. 

general  or  specific,  6. 

physical  and  chemical,  7. 

secondary,  11-18. 

Ptolemy's  system  of  astronomy,  802. 
Pulleys,  compound,  79. 

example  and  formulie,  79. 

compound,  with  one  movable  pulley,  80. 

example  and  formulie,  80. 

movable  and  immovable,  77. 

example  and  formate,  77. 

simple  fixed,  75. 

simple  movable,  with  formulie,  76. 

system  of,  with  more  than  one  cord,  78. 

example  and  formulae,  78. 

system  of,  with  more  than  one  rope  and 
three  cords  to  each  pulley,  81. 

example  and  formuloe,  81. 

the  burton,  82. 


490 


INDEX. 


Pulleys,  example  and  formulae,  82. 
Pulse-glass,  Franklin's,  319. 
Pumps,  centrifugal,  169. 

chain,  168. 

T-centrifugal,  170. 
Pumps,  rotary,  double  cog-wheel,  177. 

double  cylinder,  175. 

single  cylinder,  176. 

suction  bellows,  178. 

diaphragm,  179. 

double-acting  force,  183. 

double-acting,  with  two  valves,  185. 

force,  182. 

plunger  force,  ISO. 

principle  of,  173. 

proof  of  atmospheric  pressure  in,  174. 

single  acting,  184. 

single  cylinder,  181. 

stomach,  187. 
Pyrometers,  280. 


QUALITY  of  musical  sounds,  589. 

Quantity  and  intensity  of  electricity  in  machines. 

Quantity  and  intensity  of  electricity,  difference 
between,  732. 

Quantity  increases  with  surface,  intensity  with 
the  number  of  pairs,  733. 

Quantity  of  electricity  required  to  produce 
chemical  action  enormous,  757. 

Quantity  and  intensity  of  heat,  difference  be- 
tween, 214. 


RADIAXT  heat,  284. 

intensity  of,  286. 

mutual  exchange  of,  between  bodies,  289. 

partially  absorbed  by  medium,  287. 

transmission  of,  298. 
Radiating  power  for  heat,  296. 
Radiation  of  heat,  284. 

cooling  by,  285. 

in  vacuo,  288. 

solar,  205. 

terrestrial,  208. 
Radius  vector,  840. 

passes  over  equal  areas  in  equal  times,  841. 
Railway  illumination,  392. 
Rain,  836. 

annual  depth  of,  342. 

days  of,  341. 

distribution  of,  340. 

drops  of,  their  effects  on  parallel  rays  of 
light,  435. 

gauge,  .339. 

wh-ire  most  abundant,  340-343 
Rainbow,  explained  by  effects  of  a  drop  of  water 
on  parallel  rays  of  light,  435. 

how  we  see  the  colors  of,  from  one  position, 
4.33. 

jriinary  and  secondary,  432. 

ihe  arch  of,  434. 

width  of  the  arch,  434. 

width  of  the  primary  bow,  434. 

width  of  the  secondary  bow,  434. 

width  of  the  space  between  the  bows,  434. 
Ram,  hydraulic,  172. 
Range  of  the  human  voice,  590. 
Rays  of  light,  362. 

how  affected  by  drops  of  water,  435. 

visual,  nearly  parallel,  456. 
Reaction  and  action  equal,  65. 

of  escaping  electricity,  687. 

of  escaping  liquids.  163. 
Recomposition  of  wnite  light,  431. 
Reed  pipes,  585. 

instruments,  585. 
Reede,  arrangement  of  in  pipes,  586. 


the 


Reflecting  telescopes,  491-494. 

Reflection  of  heat  from  concave  mirrors,  292. 

incident  heat  absorbed  and  reflected,  290. 

laws  which  govern,  291. 

reflective  power  of  different  substances, 

293. 
Reflection  of  light  at  curved  surfaces,  381. 

by  convex  spherical  mirrors,  382. 

double,  of  mirrors,  398. 

of  light  at  plane  surfaces,  370. 

of  converging  rays,  371. 

of  diverging  rays,  370. 

of  parallel  rjys,  372. 

polarization  by,  515. 

of  sound,  544-554. 
Reflective  power  for  heat,  determination  of,  294. 

causes  modifying.  297. 
Reflectors,  365. 

forms  of,  367. 

paraboloid,  392. 

spherical  aberration  of,  391. 
Refraction  of  heat,  301. 

of  light,  definitions,  394. 

by  a  sphere  of  glass,  407. 

by  dense  media,  401. 

by  parallel  strata  of  different  media,  397. 

by  plane  glass,  397,  398,  406. 


by  prism*,  406. 
by  the  atn 


by  the  atmosphere,  400. 

cause  of,  396. 

depth  of   water  rendered  apparently  less 
by,  404. 

double,  519. 

effects  of,  on  the  rising  and  setting  of  heav- 
enly bodies,  400. 

index  of,  394. 

laws  of,  395. 

Refraction  and  internal  reflection,  398. 
Refraction  and  total  reflection,  399. 

of  sound,  581. 

laws  of,  582. 
Refractory  bodies,  304. 
Regulator  of  steam-engine,  354. 
Relation  of  bodies  to  light,  359. 

of    cohesion  and   repulsion  in   the  three 
states  of  matter,  23 

of  power  to  weight,  66. 
Relative  influence  of  the  sun  and  moon  on  the 

tides,  922. 
Repulsion,  electrical,  656,  657. 

in  gases,  91. 

magnetic.  625,  626. 

of  heat,  202. 

Reservoir  of  electricity,  660. 
Resistance  to  fracture,  27. 
Resolution  of  forces.  58. 
Resonance,  549. 

Respiration,    animal,    dependent  upon    atmos- 
pheric pressure,  146. 
Respiration   and  combustion    consume    oxygen 

and  supply  carbonic  acid,  124. 
Rest,  absolute  and  relative,  15. 

no  absolute,  15. 
Resultant  of  forces,  58. 

of  motion,  58. 
Retina  of  the  eye,  443. 

duration  of  impression  on,  470. 

sensibility  of,  462. 

size  of  image  on.  457. 
Reti'Ograde  motion  of  the  moon's  nodes,  906. 

of  planets,  891. 
Revolution  of  primary  planets,  815. 

diurnal,  with  table,  817. 

periodic,  with  table,  815. 

sidereal  and  synodic  of  the  moon,  856. 
Revolving  electro-magnets,  779. 

lights,  416. 
Right  ascension  of  a  body,  938. 

angle  of.  938. 
Rings  of  Saturn.  852. 

dimensions  and  distances  of,  852. 
Rivers,  flowing  of.  156. 

velocity  of,  157. 


INDEX. 


491 


Rods,  vibration  of,  576. 
Rooms  for  speaking,  553. 
Rotary  pumps,  175-177. 

steam-engines,  348. 
Rotation,  19. 


S. 

SAFETY  lamp  of  Davy,  245. 
Satellites  or  moons,  807. 

compound  motion  of,  825. 

distance   of    in    semi-diameters   of   their 
planet?,  853. 

Earth's.  826. 

Jupiter's,  with  table,  827. 

Neptune's.  830. 

Saturn's,  with  table,  828. 

Uranus1,  with  table,  829. 
Saturated  space,  323. 
Saturn's  rings,  852. 

dimensions  and  distances  of,  852. 
Saturn's  satellites,  distances  and  periodic  times, 
828. 

eclipses  of,  911. 
Scale,  musical,  596. 

formation  of,  597. 
Scissors,  a  variety  of  lever,  69. 
Screw,  endless,  86. 

combination  of  with  other  powers,  86. 

modification  of  inclined  plane,  84. 

example  and  formulae,  84. 

of  Archimedes,  171. 
Seasons,  867. 

causes  of,  868. 

of  different  planets,  878. 
Secondary  axis,  390. 

currents,  electrical,  784,  785. 

properties  of  matter,  11-24. 
Secondary  planets,  807,  825. 

compound  motion  of,  825. 
Self-registering  thermometers,  282. 
Sensible  horizon.  938. 
Series  of  elastic  balls,  65. 
Sextant,  mariner's,  380. 
Shadows,  522. 

density  of,  525. 

dimensions  of  the  earth's,  900. 

dimensions  of  the  moon's.  900. 

of  bodies  larger  than  the  illuminating  body, 
522. 

of  bodies    smaller  than  the  illuminating 
body,  523. 

of  solar  bodies,  894. 
Sidereal  revolution  of  the  moon,  856. 
Signs  of  the  zodiac,  866. 

division  of,  874. 
Sine  of  the  angle  of  incidence,  394. 

of  refraction,  394. 
Size,  apparent,  of  objects,  383. 
Smce's  battery,  735. 
Simple  microscope,  476. 

pendulum,  61. 

propositions  respecting,  61. 

vision  with  two  eyes,  467. 
Snow,  337. 
Solar  bodies,  801. 

shadows  of,  894. 

Solar  and  lunar  ecliptic  limits,  907. 
Solar  microscope.  479. 

radiation,  205. 

spectrum,  419. 

colors  of,  419. 

dark  lines  in,  426. 

properties  of,  420. 

refraction  and  dispersion  of,  425. 

and  sidereal  time,  854. 

day,  854. 

system.  807. 

centre  of  gravity  and  motion  of,  845. 

impossibility  of  delineating,  810. 

represented  by  real  objects,  810. 
Solenoid,  770. 


Solidification,  304. 

Solidification,  change  of  volume  by,  304. 

liberation  of  heat  by,  303,  304. 
Solids,  characteristic  properties  of,  25-34. 

conductivity  of,  for  heat,  240-244. 

expansion  of,  215-219. 

structure  in,  24. 

undulations  of,  576-580. 

velocity  of  sound  in,  542. 
Solstices,  869. 
Sonometer,  573. 
Sonorous  or  sounding  bodies,  531. 

difference  of  bodies.  535. 

waves,  length  of,  590. 
Sound,  533. 

a  sensation,  533. 

distance  calculated  by,  537. 

causes  which  modify  intensity  of,  556. 

from  pipes,  583. 

intensity  of,  555. 

interference  of,  564. 

limits  of  perceptible,  590. 

mediums  of,  532. 

not  instantaneous,  536. 

not  propagated  in  a  vacuum,  575. 

propagated  by  waves,  532. 

reflected,  544-554. 

refraction  of,  581. 

refraction,  laws  of,  581. 

time  required  for  transmission  of,  536. 

velocity  of,  538. 

velocity  of  in  air,  539. 

velocity  of  in  gases,  540. 

velocity  of  in  liquids,  541. 

velocity  of  in  solids,  542. 
Sounding  bodies  vibrate,  531. 
Sounds  caused  by  burning  hydrogen,  574. 

different,  534. 

qualities  of  musical,  589. 

time  required  to  distinguish,  543. 

velocity  the  same  for  all,  538. 
Sources  of  heat,  205-213. 

of  heat  influencing  diathermancy,  299. 

of  light,  357. 

Space  described  by  a  falling  body,  55. 
Space,  extent  of,  795. 
Spark,  electrical,  684,  700,  702,  768. 

color  of,  701. 

Speaking,  room  suitable  for,  553. 
Speaking-trumpet,  558. 
Specific  gravity,  114. 

instrument  for  finding  of  liquids,  116. 

method  of  finding  of  liquids,  115. 

rule  and  example,  115. 

method  of  finding  of  solids,  115. 

rule  and  example,  115. 
Specific  identity  of  matter,  7. 

heat,  231. 

affected  by  change  of  state,  '238. 

effect  of,  of  water  on  climate,  234. 

of  gases,  235. 

standard  of,  233. 

table  of,  of  different  substances,  233. 

table  of,  of  different  states  of  bodies,  2 

weight,  114. 
Spectacles,  475. 
Spectrum,  solar,  419. 

dark  lines  in,  426. 

properties  of,  420. 
Specula,  365,  366. 
Spherical  aberration  of  lenses,  412. 

of  mirrors.  391. 
Spherical  mirrors,  concave,  386-388. 

convex,  381,  382. 
Spheroidal  state  of  liquids,  331. 

cause  of,  331. 
Spirit-level,  97. 

thermometer,  281. 
Spring  and  neap  tides,  923-925. 

variations  in,  924. 
Springs,  intermittent,  99,  189. 
Spy-glass,  490. 
Stable  equilibrium,  40. 


492 


INDEX. 


Stability  of  bodies,  43. 

Stability,  dependent  upon  position  of  centre  of 
gravity,  43. 

relative  of  cubes  and  pyramids,  44. 
Stars,  binary.  944. 

clusters  of,  945. 

double,  943. 

motion  of,  940. 

the  number  of,  949. 

temporary,  or  new,  and  lost,  942. 

veriable  and  periodical,  941. 

(see  nebulie). 
Statical  electricity,  648. 
Statics,  54. 
Steam-boiler,  351. 

explosions  of,  330. 
Steam,  electricity  from,  682. 
Steam-engines,  345-354. 

boilers  of,  351. 

condensation  in,  352. 

fly-wheels  of,  354. 

governors  of,  354. 

high-pressure,  349. 

illustration  of  principle  of  low  pressure,  333. 

improvements  in,  347. 

low-pressure,  or  condensing,  354. 

origin  of,  345. 

parallel  motion  of,  354. 

reciprocating  and  rotary  motion  of,  348. 

stuffing-boxes  of,  353. 

the  eccentric,  its  importance,  350. 

the  eolipile,  846. 

valves  of,  351. 

Watt's  improvement,  347. 
Steam  heaters,  255. 
Steam,  high  pressure  of,  334. 

latent  heat  of,  314. 

latent  and  sensible  heat  of  at  different  tem- 
peratures, 315. 

Steelyards  an  example  of  levers,  69. 
Stereomonoscope,  498. 
Stereoscope,  496. 

principles  of,  497. 
Stomach-pump,  187. 
Structure,  in  solids,  24. 
Structure  of  the  human  eye,  440-444. 
Submerged  bodies  displace  water  equal  to  their 
own  bulk,  113. 

not  pressed  equally  in  all  directions,  113. 
Substance,  defined.  1. 
Suction,  explained,  132. 
Suction  and  lifting  pumps,  173-185. 
Sulphate  of  copper  battery,  786. 
Sun,  dark  spots  on,  814. 

declination  of,  871,  931. 

distance  of,  813. 

effects  of  its  declination  on  temperature, 
934. 

his  apparent  motion  in  the  ecliptic,  873. 

influence  of,  811. 

magnitude  of.  812. 

motions  of,  814. 

orbit  of,  893. 

periodic  revolution  of,  893. 

quantity  of  heat  from,  206. 

telescopic  view  of,  814. 

the  principal  source  of  heat,  205. 

velocity  of  his  motion  in  space,  893. 

when  it  shineaon  the  poles  of  the  earth, 

933. 

Synodic  revolution  of  the  moon,  856. 
Syphon,  188. 

conveying  water  over  hills  with,  191. 

dependent  on  atmospheric  pressure,  188. 

for  the  chemical  laboratory,  192. 
System,  solar,  807. 

centre  of  gravity  and  motion  of,  845. 
Systems,  binary,  of  stars,  944. 

T. 

TABLE  of  absolute  number  of  vibrations  corre- 
sponding to  musical  notes,  597. 


Table  of  asteroids,  824. 

boiling    point    at    different    atmospheric 
pressures,  334. 

conductibility  of  solids,  242. 

depth  of  rain,  342. 

discharge  of  liquids,  155. 

expansion  of  liquids,  222. 

expansion  of  solids,  219. 

falling  bodies,  55. 

frequency  of  different  winds,  268. 

length  of  waves  of  light.  509. 

melting  points  of  different  substances,  304. 

specific  heat  of  different  states  of  bodies, 
238. 

specific  heat  of  different  substances,  233. 

velocity  and  force  of  winds,  270. 
Tables  relating  to  primary  planets,  815-824. 

eccelitricity  of  orbits,  838. 

inclination  of  orbits  to  plane  of  ecliptic, 
849. 

polar  inclination,  878. 

secondary  planets,  826-829. 
Telegraph,  first  experiments  in  electrical,  776. 

House's  printing.  777. 

Morse's  recording,  777. 

the  earth  circuit,  778. 

Watson's,  776. 
Telegraphic  alphabet,  777. 
Telescopes,  different  kinds  of,  488-494. 

Galileo's,  481. 

Gregorian  reflecting,  492. 

Lord  Rosse's  reflecting,  494. 

Newtonian  reflecting,  493. 

retracting  astronomical,  489. 

Sir  William  Herschel's  reflecting.  491. 

terrestrial,  490. 
Telescopic  views  of  the  primary  planets,  836. 

of  the  sun,  814. 
Telestereoscope,  495. 
Temper,  83. 
Temperature,  200. 

extremes  of  natural,  207. 

measurement  of,  271. 

of  climate  affected  by  declination  of  the 
sun,  934. 

of  plants,  213. 

Tenacity  or  resistance  of  substances,  27. 
Tenacious  liquids,  fall  or  flow  of,  56. 
Tension  of  vapors,  323. 

cold  diminishes  and  heat  increases,  323, 325. 

maximum,  323. 

electrical,  662. 
Terrestrial  attraction,)  18. 

direction  of,  53. 

globes,  937. 

heat,  origin  of,  208. 

magnetism,  624,  633,  638,  643. 
Theoretical  and  actual  flow,  153. 
Theory,  Ampere's,  relating  to  electro-magnetism, 
766. 

Copernicus',  of  astronomy,  803. 

electro-chemical,  725. 

of  heat,  emission,  201. 

of  heat,  undulatory,  201. 

of  light,  emission,  356. 

of  light,  undulatory,  356. 

of  magnetism,  625. 

Ptolemy's,  of  astronomy,  802. 

single-fluid,  of  electricity,  655. 

two-fluid,  of  electricity,  654. 

wave  of  light,  499. 

Volta's,  contact  of  Galvanism,  724. 
Thermo-electric  revolving  arch,  788. 
Thermo-electricity,  787. 
Thermometers,  271. 

Centigrade,  Fahrenheit,  Reaumur,  272. 

conversion  of  scales  of,  273. 

differential,  283. 

fixing  the  boiling  point,  277. 

fixing  the  freezing  point,  276. 

limits  of  mercurial,  280. 

mercurial,  272. 

method  of  making,  274. 


INDEX. 


493 


Thermometers,  method  of  graduating,  276. 

self-registering,  282. 

sensibility  of,  279. 

spirit,  281. 

standard  points  in,  275. 

tests  of,  278. 

Three  states  of  matter,  23. 
Thunder,  713. 
Thunder  storms,  712. 

clouds,  origin  of,  712. 
Tides,  atmospheric,  930. 

affected  by  winds,  926. 

affected  by  conformation  of  land,  927. 

average  elevation  of.  928. 

different  heights  of  in  different  oceans  and 
seas,  929. 

influence  of  the  sun  on,  919. 

lagging  of  the  tide-wave,  918. 

not  uniform,  913. 

opposite  tide-wave,  cause  of,  920. 

principal  cause  of,  914. 

relative  influence  of  the  sun  and  moon  on, 
922. 

secondary  cause  of  opposite  tide-wave,  921. 

single  tide-wave,  916. 

spring  and  neap,  923-925. 

two  tide- waves,  917. 

variations  in  spring  and  neap,  924. 
Time,  equation  of,  854. 

required  for  distinguishing  sounds,  543. 

for  transmission  of  sound,  536. 

for  vision,  472. 

periodic,  of  heavenly  bodies,  815. 

solar  and  sidereal,  854. 
Tone  changed  by  echo,  548. 
Tornadoes,  266. 
Torsion,  26. 
Total  reflection,  399. 

Tourmaline,  action  of  on  ordinary  light,  514. 
Toys,  optic,  471. 
Trade-winds,  262. 
Transits,  879. 

calculation  of,  881. 

of  Mercury,  880. 

of  Venus,  892. 

Translation  or  direct  motion,  19. 
Translucent  bodies,  359. 
Transmission  of  luminous  waves,  513. 

of  radiant  heat,  298. 

of  sound,  time  required  for,  536. 
Transparent  bodies,  359. 
Trumpet,  ear.  558. 

speaking,  559. 
Tubes,  capillary,  38. 
Tuning-fork,  564. 
Turbine  wheel,  162. 

U. 

UMBRA,  524. 

Undershot  wheel,  161. 

Undulations,  combinations  of  in  liquids,  565. 

interference  of,  of  liquids  in  an  ellipse,  566. 

of  elastic  fluids,  567. 

of  light,  499. 

of  solids,  569. 
Uniform  motion,  54. 
Unison,  591. 
Unit  of  force,  54. 
Unit  of  heat,  232. 
Universe,  950. 
Universal  discharger,  697. 
Unstable  equilibrium,  40,  42. 
Up  and  down,  relative  terms,  53. 
Upward  pressure  of  atmosphere,  126. 

of  liquids.  100. 

Cranus'  satellites,  distances  and  periodic  times 
of,  829. 


VACUUM,  evaporation  in,  323. 
fountain  in,  145. 


Vacuum,  various  phenomena  in,  129. 
Valves,  operation  of  in  steam-engine,  351. 

safety,  of  steam-boilers,  351. 
Vaporization,  definitions,  311. 
Vapors  and  gases,  identity  of,  119. 

condensation  of  vapors,  327. 

density  of,  229. 

formed  in  a  vacuum,  323. 

tension  of,  119. 

tension,  maximum  of,  323. 
Variable  motion,  54. 
Variations  of  barometric  height,  135. 

of  the  needle,  638-640. 
Varieties  of  motion,  19. 
Vegetables,  electricity  of,  791. 

temperature  of.  213. 
Velocity,  accelerated,  of  falling  bodies,  55,  56. 

of  comets,  832. 

discharge  of  liquids,  155. 

electricity,  716. 

heavenly  bodies,  800. 

jets,  154. 

light,  526. 

lightning,  716. 

rivers,  156. 

planets,  with  table,  816. 

sound  in  air,  538,  539. 

sound  in  gases,  540. 

sound  in  liquids,  541. 

sound  in  solids,  542. 
Venus  as  morning  and  evening  star,  851. 

transits  of,  892. 

transits,  list  of,  892. 
Vibrating  cords,  nodal  points  of,  571. 
Vibration  of  cords,  570-573. 

laws  of,  572. 

verification  of  laws  of,  573. 
Vibration  of  air  in  pipes,  583. 
Vibrating  rods  and  plates,  nodal  points  and  lines 

of,  578. 
Vibration  of  plates.  577. 

of  rods,  576. 

Vibrations,  absolute  number  of  corresponding 
to  musical  notes,  597. 

cause  of  in  sonorous  bodies  illustrated  by 
striking  a  bell,  561.  562. 

of  light,  499. 

of  light,  direction  of,  500. 

of  sonorous  bodies,  531. 

of   sonorous  bodies,  illustrated  by  Jews- 
harp,  560. 

'    transverse,  of  light,  500. 
View  of  the  earth  from  the  moon,  861. 

of  the  moon  from  the  poles  and  equator  of 

the  earth,  884. 
Views,  dissolving,  480. 

microscopic.  479. 

telescopic,  of  the  planets,  836. 

of  the  sun,  814. 
Virtual  focus,  388. 

Visible  bodies  emit  licht  from  every  point,  363. 
Vision,  439. 

anele  of,  448. 

binocular,  469. 

brilliancy,  454. 

conditions  of  distinct,  461. 

double,  468. 

how  we  see  objects  close  to  the  eye,  453. 

indistinct.  452. 

limits  of  distinct,  455. 

sensations  of  excited  by  other  causes  than 
light,  473. 

time  required  to  produce,  472. 

why  we  see  objects  erect,  their  images  be- 
ing inverted,  450. 
Visual  rays,  nearly  parallel.  456. 
Vocal  apparatus  of  man,  587. 
Voice,  range  of  human,  590. 
Volatile  and  fixed  liquids,  312. 
Voltaic  arch,  747. 

heat  of,  751. 

oval  form  of,  748. 
Voltaic  batteries,  727-740. 


494 


INDEX. 


Voltaic  circuit,  760. 

polarity  of,  729,  760. 
Voltaic  currents,  730. 

decomposition  of  water  by,  755. 

decomposition  of  salts  by,  756. 
Voltaic  electricity,  745-758. 

chemical  effects  of,  752. 

heating  effects  of,  751. 

illuminating  effects  of,  740. 

physiological  effects  of,  758. 

quantity  and  intensity  of,  732. 
Voltaic  pile  or  battery,  727. 

chemical  effects  of,  752,  753. 

chemical  theory  of,  725. 

electrical  currents  of,  730. 

grouping  elements  of,  727. 

heating  effects  of,  751. 

magnetic  effects  of,  761. 

physical  effects  of,  746. 

physiological  effects  of,  758. 

polarity,  729. 

theory  of,  724. 

varieties  of,  728. 

simple  couple,  726. 
Voltaic  spark  and  arch,  747. 

oval  form  of,  748. 
Voltaism  and  galvanism,  721-724. 
Volta's  contact  theory,  724. 

discoveries,  724. 


W. 

WARMING  buildings  by  convection  of  air,  256. 

by  convection  of  fluids  in  pipes,  255. 

by  steam  in  pipes,  257. 
Water  as  a  motive  power,  158. 

an  exception  to  the  laws  of  contraction  and 
expansion,  223. 

beneficial  effects  of  unequal  expansion  of, 

boiling  temperature  of,  317. 

composition  of,  755. 

compressibility  of,  89. 

conveyed  over  hills  by  siphons,  191. 

decomposition  of,  755. 

elasticity  of,  89. 

expands  in  freezing,  223. 

its  flow  in  rivers,  156. 

freezing  of  in  small  tubes,  225. 

freezing  point,  temperature  of,  223. 

great  capacity  of  for  heat,  233. 

how  heated,  253. 

illustrations  of  the  pressure  of,  100-113. 

importance  of,  197. 

importance  of  elevators  of,  197. 

level  of,  94-98. 

loss  of  effective  head  of,  193. 

pressure  of  at  different  depths,  110-112. 

specific  heat  of,  233. 

standard  of  specific  heat,  233. 

velocity  of  in  pipes,  how  retarded,  190. 

velocity  of  in  rivers.  157. 

velocity  of  discharge  of,  155. 

vertical  jets  of,  195. 

why  rises  by  suction,  132,  173,  174. 

why  rises  in  pumps,  174. 
Water  elevators,  Archimedes'  screw,  171. 

centrifugal  pump,  169. 

chain  pump,  168. 

endless  chain  of  pots,  167. 

hydraulic  ram,  172. 

T-centrifugal  pnmp,  170. 

lifting  wheel,  165. 

wheel  and  buckets,  or  Persian  wheel,  166. 
Water  level,  96. 

pumps,  173-186. 
spouts,  266. 


Water  wheels,  158. 

breast,  160. 

centrifugal,  163. 

overshot,  159. 

turbine,  162. 

undershot,  161. 
Waves  of  condensation  and  rarefaction,  567. 
Waves  of  light,  356,  499. 

brilliancy  dependent  on  amplitude  of,  501. 

causes  of,  510. 

color  dependent  on  length  of,  502. 

determining  the  length  of,  508. 

direction  of,  500. 

in   any  number  of  planes  resolved  to   two 
planes,  517. 

length  of,  509. 

luminous,  transmission  of,  513. 

table  of  length  of,  509. 
Waves  of  liquids,  combinations  of,  565. 

from  foci  of  an  ellipse,  566. 

interference  of,  565. 

interference  of  in  an  ellipse,  566. 
Waves,  reflection  of  from  parabolic  curves,  554. 
Waves  of  sound,  caused  by  striking  a  bell,  561. 

interference  of,  568. 

sonorous,  co-existence  of,  568. 

tide,  916-921. 
Weather  indicated  by  barometer,  136. 

rules  for  judging  by  the  barometer,  136. 
Wedge,  a  form  of  inclined  plane,  85. 

formulae  respecting,  85. 
Weight,  definition  of,  39. 

different  in  different  localities,  48-51. 

as  resistance,  66. 
Welding,  34. 
Wells,  artesian,  95. 
Wheel  and  axle,  71. 

example  and  formulae,  71. 

compound,  74. 

example  and  formulae.  74. 

barometer,  138. 
Whirlwind,  266. 
Whispering  galleries,  552. 
White  light,  recomposition  of,  431. 
Windlass,  simple,  72. 

differential  or  double,  73. 
Winds,  action  of  on  sails,  59. 

cause  of,  262. 

definition  of,  260. 

general  direction  of  frequency  of,  268. 

hurricanes  or  cyclones,  265. 

kinds  of,  261. 

land  and  sea  breezes,  264. 

periodical,  261. 

physical  properties  of,  267. 

pressure  of,  269. 

regular,  261. 

table  respecting,  270. 

tornadoes  or  whirlwinds,  266. 

trade,  262. 

variable,  261. 

velocity  of,  270. 

Wood,  conduction  of  heat  by,  244. 
Working  point  in  machinery,  66. 

Z. 

ZERO  absolute,  275. 

Zenith  distance,  938. 

Zero  point  of  thermometers,  272. 

Zodiac,  865. 

names  of  the  signs  of,  866. 

signs  or  constellations  of,  866,  872. 
Zones,  932. 

frigid,  832. 

temperate,  832. 

table  of,  of  different  planets,  878. 

torrid,  932. 


YC  49592 


541752 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


